Python at length

Understanding variables

Variables are simply declarations that are used to store certain values. For instance, the variable name can hold the value of John Smith. Several rules need to be considered when declaring variable names. For starters, a variable name cannot begin with a number.

2name = incorrect #incorrect

name = correct #correct

Variable names are case sensitive. This means that the variable school is not the same as School.

Variables can hold different data types. This includes strings, integers, Booleans, long, lists, and arrays.

In Python, we do not need to declare the data type while writing a variable. This is because the code is compiled and interpreted later. The compiler will throw an error in case there is a mismatch in the data types.

Let's talk about the different data types.

1. Strings

Strings are usually presented in a text format. We will declare a string variable, as shown below.

name = "john"school = "Alliance Francaise"

When we run print(name), the output will be john.

1. Integers

These variables hold numeric values, as shown below.

math = 90chemistry = 100biology = 70

We can find the total of the variables above using the following statement.

print(math+chemistry+biology)

The total is 260.

A TypeError is thrown when you try to add a string to an integer, as shwon below.

var1 = "30" #stringvar2 = 20 #integer​print(var1+var2)#type error

We can sum var1 and var2 by converting var1 to an integer using the int() function. The following code will execute successfully.

var1 = "30" #stringvar2 = 20 #integer​print(int(var1)+var2) # Output: 50

Make sure that the variable stores a value that can be converted to an integer before using the int() method.

1. Booleans

There are only two Boolean values: True and False. In other words, something can either be true or false. We declare these values, as shown below. Please note that Python is case sensitive.

isOn = TrueisChecked = False

A bool() method can help convert a value to a boolean. The code snippets below showcase how a bool() function can be used.

print(bool("abc")) #returns Trueprint(bool(0))  #returns False

The bool() function returns False when there are no parameters.

1. Float

This data type consists of numbers that have a decimal place. A perfect example of a float variable is highlighted below.

Bmi = 45.7

Understanding lists

Lists allow us to store numerous elements in a particular variable. For instance, we can have a list that stores all the student names in a class. We use [] to define a list.

students = [] #list example

Elements in a list are usually separated by a comma, as shown below.

students = ["john", "Mary Thomas", "John Smith"]

Each element in the above students list has an index. By default, the first index is 0. So the item at index [0] is john, while the value at index 1 is Mary Thomas. A list of integers will look as follows.

student_marks = [90, 78, 90, 78]

We can access different list functionalities using built-in functions. For instance, to add a value to the student_marks list, we use the append function.

student_marks.append("Guardian Angel")print(student_marks)

The above function adds Guardian Angel at the end of the student_marks list.

When we print the list it shows:

#output[90, 78, 90, 78, 'Guardian Angel']

We use len(student_marks) to determine the length of the list. We use the remove() function to delete something from the list. For instance, we can remove 90 from the student_mark list as shown below.

student_marks.remove(90)print(student_marks)

In lists, negative indices allow us to count elements starting from the last one. For instance, the element with an index of -1 in the above student_marks list is "Guardian Angel". The second last element 78 has an index of -2.

Understanding functions or methods

Methods are quite critical in programming. They help store reusable code. This means that a person can call already declared methods rather than writing statements from scratch repeatedly. This saves significant time, that can be invested in other productive activities.

In Python, we use the def keyword to declare a function. An example of a python method is shown below.

def readData():    print('success')

The above function prints success when it's invoked. We can also pass data to a method, perform some calculations, and return the results. This is demonstrated in the code snippet below.

def calculateTotal(chem, bio):    return chem+bio​print(calculateTotal(90,80))

The calculateTotal method takes in two parameters (chem, bio). The function then returns the sum of the two values. It is important to take note of the data types when passing parameters. For instance, the calculateTotal method will not work when we pass in a string as a parameter. This is because the program cannot sum up an integer and a string. As shown above, we can call the calculateTotal method directly from our print statement.

print(calculateTotal(90,80))

The return keyword ensures that the method returns a result after execution.

Note that a function can also call another method. This is illustrated below.

def readData(chem, bio):    return chem+bio​def getTotal():    print(readData(90,80)) #calls the readData method​getTotal()

Understanding loops

Loops are critical because they allow us to iterate through lists, check for different conditions, and continuously execute various statements. The main loops are for and while.

1. For loops As noted, we can use a for loop to iterate through a list, as shown below:

student_list = ["John Doore","Matu Smith"]for x in student_list:    print(x)

The for loop above will print every item in the student_list.

1. While loops A while loop can help us check for a particular condition. For instance, while something is true specific statements can be executed. Here is an example of a while loop in action.

isChecked = falsewhile isChecked == true:    print('Hallo there')

Note that the while loop above will be executed indefinitely until isChecked is set to false. You can press ctrl+c to stop the loop.

Classes

Classes are a vital component of object-oriented programming. When creating a class, you must use the class keyword. Other elements are then nested in the class. Here is an example of a Python class.

class Farmer: # a class with the name farmer    name = "John" # A variable    produce = "1000kgs" # A variable​farmer = Farmer() #instatiating the class as an object. print(farmer.name) # accessing the properties of the Farmer class.

Classes can help as group things with similar characteristics. We can also assign values to class variables using the init function.

class Farmer:  def __init__(self, farmername, produce):    self.farmername = farmername    self.produce = produce​farmer = Farmer("Carry Sminson", "10,000kgs")​print(farmer.farmername, farmer.produce)

In the above Farmer class, the self keyword represents an instance of an object. In other words, it allows us to access the different methods and attributes defined in the class.

You can also declare a method in a class and use it later, as shown below.

class Farmer:  def __init__(self, farmername, produce):    self.farmername = farmername    self.produce = produce​  def printDetails(self): # Method      print(self.farmername, self.produce)​farmer = Farmer("Carry Sminson", "10,000kgs")​farmer.printDetails()

Python syntax was made for readability, and easy editing. For example, the python language uses a : and indented code, while javascript and others generally use {} and indented code.

Lets create a [python 3](https://repl.it/languages/python3) repl, and call it Hello World. Now you have a blank file called main.py. Now let us write our first line of code:

helloworld.py

print('Hello world!')

Brian Kernighan actually wrote the first "Hello, World!" program as part of the documentation for the BCPL programming language developed by Martin Richards.

Now, press the run button, which obviously runs the code. If you are not using replit, this will not work. You should research how to run a file with your text editor.

If you look to your left at the console where hello world was just printed, you can see a >, >>>, or $ depending on what you are using. After the prompt, try typing a line of code.

Python 3.6.1 (default, Jun 21 2017, 18:48:35)[GCC 4.9.2] on linuxType "help", "copyright", "credits" or "license" for more information.> print('Testing command line')Testing command line> print('Are you sure this works?')Are you sure this works?>

The command line allows you to execute single lines of code at a time. It is often used when trying out a new function or method in the language.

Another cool thing that you can generally do with all languages, are comments. In python, a comment starts with a #. The computer ignores all text starting after the #.

shortcom.py

# Write some comments!

If you have a huge comment, do not comment all the 350 lines, just put ''' before it, and ''' at the end. Technically, this is not a comment but a string, but the computer still ignores it, so we will use it.

longcom.py

'''Dear PYer,I am confused about how you said you could use triple quotes to makeSUPERLONGCOMMENTS!​I am wondering if this is true,and if so,I am wondering if this is correct.​Could you help me with this?​Thanks,Random guy who used your tutorial.'''print('Testing')

Unlike many other languages, there is no var, let, or const to declare a variable in python. You simply go name = 'value'.

vars1.py

x = 5y = 7z = x*y # 35print(z) # => 35

Remember, there is a difference between integers and strings. Remember: String = "". To convert between these two, you can put an int in a str() function, and a string in a int() function. There is also a less used one, called a float. Mainly, these are integers with decimals. Change them using the float() command.

vars2.py

x = 5x = str(x)b = '5'b = int(b)print('x = ', x, '; b = ', str(b), ';') # => x = 5; b = 5;

Instead of using the , in the print function, you can put a + to combine the variables and string.

There are many operators in python:

• +
• -
• /
• * These operators are the same in most languages, and allow for addition, subtraction, division, and multiplicaiton. Now, we can look at a few more complicated ones:
• %
• //
• **
• +=
• -=
• /=
• *= Research these if you want to find out more…

simpleops.py

x = 4a = x + 1a = x - 1a = x * 2a = x / 2

You should already know everything shown above, as it is similar to other languages. If you continue down, you will see more complicated ones.

complexop.py

a += 1a -= 1a *= 2a /= 2

The ones above are to edit the current value of the variable. Sorry to JS users, as there is no i++; or anything.

Fun Fact: The python language was named after Monty Python.

If you really want to know about the others, view [Py Operators](https://www.tutorialspoint.com/python/pythonbasicoperators.htm)​

Like the title? Anyways, a ' and a " both indicate a string, but do not combine them!

quotes.py

x = 'hello' # Goodx = "hello" # Goodx = "hello' # ERRORRR!!!

slicing.py

String Slicing

You can look at only certain parts of the string by slicing it, using [num:num]. The first number stands for how far in you go from the front, and the second stands for how far in you go from the back.

x = 'Hello everybody!'x[1] # 'e'x[-1] # '!'x[5] # ' 'x[1:] # 'ello everybody!'x[:-1] # 'Hello everybod'x[2:-3] # 'llo everyb'

Methods and Functions

Here is a list of functions/methods we will go over:

• .strip()
• len()
• .lower()
• .upper()
• .replace()
• .split()

I will make you try these out yourself. See if you can figure out how they work.

strings.py

x = " Testing, testing, testing, testing       "print(x.strip())print(len(x))print(x.lower())print(x.upper())print(x.replace('test', 'runn'))print(x.split(','))

Good luck, see you when you come back!

Input is a function that gathers input entered from the user in the command line. It takes one optional parameter, which is the users prompt.

inp.py

print('Type something: ')x = input()print('Here is what you said: ', x)

If you wanted to make it smaller, and look neater to the user, you could do…

inp2.py

print('Here is what you said: ', input('Type something: '))

Running: inp.py

Type something:Hello WorldHere is what you said: Hello World

inp2.py

Type something: Hello WorldHere is what you said: Hello World

Python has created a lot of functions that are located in other .py files. You need to import these modules to gain access to the,, You may wonder why python did this. The purpose of separate modules is to make python faster. Instead of storing millions and millions of functions, , it only needs a few basic ones. To import a module, you must write input <modulename>. Do not add the .py extension to the file name. In this example , we will be using a python created module named random.

module.py

import random

Now, I have access to all functions in the random.py file. To access a specific function in the module, you would do <module>.<function>. For example:

module2.py

import randomprint(random.randint(3,5)) # Prints a random number between 3 and 5

Pro Tip: Do from random import randint to not have to do random.randint(), just randint() To import all functions from a module, you could do from random import *

Loops allow you to repeat code over and over again. This is useful if you want to print Hi with a delay of one second 100 times.

for Loop

The for loop goes through a list of variables, making a seperate variable equal one of the list every time. Let's say we wanted to create the example above.

loop.py

from time import sleepfor i in range(100):     print('Hello')     sleep(.3)

This will print Hello with a .3 second delay 100 times. This is just one way to use it, but it is usually used like this:

loop2.py

import timefor number in range(100):     print(number)     time.sleep(.1)

while Loop

The while loop runs the code while something stays true. You would put while <expression>. Every time the loop runs, it evaluates if the expression is True. It it is, it runs the code, if not it continues outside of the loop. For example:

while.py

while True: # Runs forever     print('Hello World!')

Or you could do:

while2.py

import randomposition = '<placeholder>'while position != 1: # will run at least once    position = random.randint(1, 10)    print(position)

The if statement allows you to check if something is True. If so, it runs the code, if not, it continues on. It is kind of like a while loop, but it executes only once. An if statement is written:

if.py

import randomnum = random.randint(1, 10)if num == 3:     print('num is 3. Hooray!!!')if num > 5:     print('Num is greater than 5')if num == 12:     print('Num is 12, which means that there is a problem with the python language, see if you can figure it out. Extra credit if you can figure it out!')

Now, you may think that it would be better if you could make it print only one message. Not as many that are True. You can do that with an elif statement:

elif.py

import randomnum = random.randint(1, 10)if num == 3:    print('Num is three, this is the only msg you will see.')elif num > 2:    print('Num is not three, but is greater than 1')

Now, you may wonder how to run code if none work. Well, there is a simple statement called else:

else.py

import randomnum = random.randint(1, 10)if num == 3:    print('Num is three, this is the only msg you will see.')elif num > 2:    print('Num is not three, but is greater than 1')else:    print('No category')

So far, you have only seen how to use functions other people have made. Let use the example that you want to print the a random number between 1 and 9, and print different text every time. It is quite tiring to type:

Characters: 389

nofunc.py

import randomprint(random.randint(1, 9))print('Wow that was interesting.')print(random.randint(1, 9))print('Look at the number above ^')print(random.randint(1, 9))print('All of these have been interesting numbers.')print(random.randint(1, 9))print("these random.randint's are getting annoying to type")print(random.randint(1, 9))print('Hi')print(random.randint(1, 9))print('j')

Now with functions, you can seriously lower the amount of characters:

Characters: 254

functions.py

import randomdef r(t):     print(random.randint(1, 9))     print(t)r('Wow that was interesting.')r('Look at the number above ^')r('All of these have been interesting numbers.')r("these random.randint's are getting annoying to type")r('Hi')r('j')

Chapter 01 - Getting Ready with Python

Installing Python 3, And Launching Python Shell

This video should help you get up and running with Python 3

• ​[Installing Python 3 and Launch Python Shell](https://www.youtube.com/watch?v=Ji1WW4Suaww)​

Installing Python is really a cakewalk. Search for "Python download" on [www.google.com](http://www.google.com/). Download the installable and install it.

A quick word of caution on Windows

• Make sure that you have the check-box "Add Python 3.6 to PATH", checked.

Once you have installed Python, you can launch the Python Shell.

• Windows - Launch cmd prompt by typing in 'cmd' command.
• Mac or Linux - Launch up terminal.

Command to launch Python 3 is different in Mac.

• In Mac, type in python3
• In other operating systems, including windows, type python

You can type code in python shell and code as well!

You can use print(5*4), and it shows 20.

You can execute the code, and the shell would immediately give you output.

Using the the Python Shell is an awesome way to learn Python.

Chapter 02 - Introduction To Python Programming

Most programmers find programming a lot of fun, and besides, it also gets their work done.

Programming mainly involves problem solving, where one makes use of a computer to solve a real world problem.

During our journey here, we will approach programming in a very different way. We will not only introduce you to the Python language, but also help you pick up essential problem solving skills.

As a programmer, you need to be able to look at a problem, and identify the important programming concepts relevant to solving it. Finally, you need to be able to use the language features and syntax, to express your solution on the computer. While all this looks complex, we want to make it easy for you. Together, we will tackle a variety of programming challenges, using these same steps. We will start with simple challenges (such as a Multiplication Table), and gradually increase the difficulty level over the duration of this book.

Learning to program is a lot like learning to ride a bicycle. The first few steps are the most challenging ones.

Once you get over these initial steps, your experience will become more and more enjoyable.

Are you ready for your first programming challenge? Let's get going now! We wish you all the best.

Summary

In this step, we:

• Were introduced to the concept of problem solving
• Understood how good programmers approach problem solving

Step 01: Our First Programming Challenge

Our first programming challenge aims to do, what every kid does in math class: read out a multiplication table. We now want to give this task to the computer. Here is the statement of our problem:

The Print Multiplication Table Challenge (PMT-Challenge)

1. Compute the multiplication table for 5, with entries from 1 to 10.
2. Display this table.

The display needs to be:

5 * 1 = 5

5 * 2 = 10

5 * 3 = 15

5 * 4 = 20

5 * 5 = 25

5 * 6 = 30

5 * 7 = 35

5 * 8 = 40

5 * 9 = 45

5 * 10 = 50

This is the challenge. For convenience, let's give it a label, say PMT-Challenge. What would be the important concepts we need to learn, to solve this challenge? The following list of concepts would be a good starting point:

• Statements
• Expressions
• Variables
• Literals
• Conditionals
• Loops
• Methods

In the rest of this chapter, we will introduce these concepts to you, one-by-one. We will also show you how learning each concept, takes us closer to a solution to PMT-Challenge.

Summary

In this step, we:

• Stated our first programming challenge
• Identified what programming concepts we need to learn, to solve this challenge

Step 02: Breaking Down PMT-Challenge

Typically when we do programming, we have problems. Solving the problem typically need a step-by -step approach. Common sense tells us that to solve a complex problem, we break it into smaller parts, and solve each part one by one. Here is how any good programmer worth her salt, would solve a problem:

• Simplify the problem, by breaking it into sub-problems
• Solve the sub-problems in stages (in some order), using the language
• Combine these solutions to get a final solution

The PMT-Challenge is no different! Now how do we break it down, and where do we really start? Once again, your common sense will reveal a solution. As a first step, we could get the computer to calculate say, 5 * 3. The second thing we can do, is to try and print the calculated value, in a manner similar to 5 * 3 = 15. Then, we could repeat what we just did, to print out all the entries of the 5 multiplication table. Let's put it down a little more formally:

Here is how our draft steps look like

• Calculate 5 * 3 and print result as 15
• Print 5 * 3 = 15 (15 is result of previous calculation)
• Do this ten times, once for each table entry (going from 1 to 10)

Let's start with that kind of a game plan, and see where it takes us.

Summary

In this step, we:

• Learned that breaking down a problem into sub-problems is a great help
• Found a way to break down the PMT-Challenge problem

Step 03: Introducing Operators And Expressions

Let's focus on solving the first sub-problem of PMT-Challenge, the numeric computation. We want the computer to calculate 5 * 5 for example, and print 25 for us. How do we get it to do that? That's what we would be looking at in this step.

Snippet-01: Introducing Operators

Launch up Python shell. We want to calculate 5 * 5. How do we do that?

Using our knowledge of school math, let's try 5 X 5.

    >>> 5 X 5    File "< stdin >", line 1    5 X 5      ^    SyntaxError: invalid syntax

The Python Shell hits back at us, saying "invalid syntax". This is how Python complains, when it doesn't fully understand the code you type in. Here, it says our code has a "SyntaxError".

The reason why it complains, is because 'X' is not a valid operator in Python.

The way you can do multiplication is by using the '*' operator .

"5 into 5" is achieved by the code 5 * 5, and you can see the result 25 being printed. Similarly, 5 * 6 gives us 30.

    >>> 5 * 6    30

There are a wide range of other operators in Python:

• 5 + 6 gives a result of 11.

• 5 - 6 leads to -1.

  >>> 5 + 611>>> 5 - 6-1

10 / 2, gives an output of 5.0 . There is one interesting operator, **. Let's try 10 ** 3. We ran this code, and the result we get is 1000. Yes you guessed right, the operator performs "to the power of". "10 to the power of 3" is 10 * 10 * 10, or 1000.

    >>> 10 / 2    5.0    >>> 10 ** 3    1000

Another interesting operator is %, called "modulo", which computes the remainder on integer division. If we do 10 % 3, what is the remainder when 10 is divided by 3? 3 * 3 is 9, and 10 - 9 is 1, which is what % returns in this case.

Let's look at some terminology:

• Whatever pieces of code we gave Python shell to run, are called expressions. So, 5 * 5, 5 * 6 and 5 - 6 are all expressions. An expression is composed of operators and operands.
• In the expression 5 * 6, the two values 5 and 6 are called operands, and the * operator operates on them.
• The values 5 and 6 are literals, because those are constants which cannot be changed.

The cool thing about Python, is that you can even have expressions with multiple operators. Therefore, you can form an expression with 5 + 5 + 5, which evaluates to 15. This is an expression which has three operands, and two + operators. You can even have expressions with different types of operators, such as in 5 + 5 * 5.

    >>> 5 + 5 + 5    15    >>> 5 + 5 * 5    30

Try and play around with the expressions, and understand the output which results.

Summary

In this step, we:

• Learned how to give code input to the Python Shell
• Understood that Python has a predefined set of operators
• Used a few types of basic operators and their operands, to form expressions

Step 04: Programming Exercise IN-PE-01

At this stage, your smile tells us that you enjoy evaluating Python expressions. What if we tickle your mind a bit, to make sure it hasn't fallen asleep? Here is your first programming exercise.

Exercises

1. Write an expression to calculate the number of minutes in a day.
2. Write an expression to calculate the number of seconds in a day.

Note

You need to solve these problems by yourself. If you are able to work them out, that's fantastic! But if not, that's part of the learning process.

Solutions

Solution 1

    >>> 24 * 60​    1440

We wanted to calculate the number of minutes in a day. How do we do that? Think about this…

• How many number of hours are there in a day? 24.
• And how many minutes does each hour have? It's 60.
• So if you want to find out the number of minutes in a day, it's 24 * 60, which is 1440.

Solution 2

    >>> 24 * 60 * 60​    86400

How many seconds are there in a day?

• Let's start with the number of hours, 24.
• The number of minutes in an hour is 60, and
• The number of seconds in a minute is 60 as well.
• So it's 24 * 60 * 60, or 86400.

Summary

In this step, we:

• Solved a Programming Exercise involving common scenarios, using Python code involving:

  • Expressions
  • Operators
  • Literals

Step 05: Puzzles On Expressions

Let's look at a few puzzles related to expressions, in this step. Before that, let's revise some of the terminology we had learned earlier.

5 + 6 + 10 is an example of an expression. In this expression, 5, 6 and 10 are operands. The + here is the operator. You can have multiple operators in an expression. We also did mention that the operands, namely 10, 6 and 5, are literals. Their values will not change.

Here are a few puzzles coming up, to explore aspects of expressions.

Snippet-01: Puzzles On Expressions

Think about what would happen when you do something of this kind: 5 $ 2. You're right, it would throw a SyntaxError. When Python does not understand the code you type in, it reports an error. Here, the expression we're typing is 5 $ 2, which does not make sense to Python, hence the SyntaxError.

    >>> 5 $ 2    File "< stdin >", line 1    5 $ 2    ^    SyntaxError: invalid syntax    >>> 5$2    File "< stdin >", line 1    5 $ 2    ^    SyntaxError: invalid syntax

Let's say we type in 5+6+10, without any spaces between the operands, and the operators. What do you think will happen? Surprisingly, the Python Shell does calculate the value!

    >>> 5+6+10    21

In an expression, using spaces makes it easier for you to read it, but it's not mandatory. 5 + 6 + 10 is easier to read than 5+6+10, but does not make any difference to the Python compiler.

The next puzzle tries to evaluate 5 / 2, which is "5 divided by 2". What would be the output? 2.5.

    >>> 5/2    2.5

If you're coming from other programming languages like Java or C, this might be a surprising result. If you try this in Java for instance, you would get 2 as the output. Note that even though both operands are integers, the result of the / operation is a floating point value, 2.5 . Python does what is expected by a programmer!

The puzzle after that tries to play with 5 + 5 * 6. What would be the result of this expression? Will it be 5 + 5 or 10, then 10 * 6, which is 60? Or, will it be 5 plus 5 * 6, which is 5 + 30, that's 35?

    >>> 5 + 5 * 6    35

The correct result is 35.

Python decides this is based on the precedence of operators.

Operators in Python are divided into two sets as follows:

• **, *, / and % have higher precedence, or priority.
• + and - have a lower precedence.

Sub-expressions involving operators from {*, /, %, **} are evaluated before those involving operators from {+, -}

Let's try another small puzzle on precedence, with 5 - 2 * 2. What would be the result of this? Will it be 6, or 1? It's 1, because * has a higher precedence than -. Thus 2 * 2 is 4, and 5 - 4 gives us 1.

    >>> 5 - 2 * 2    1

Let's say we want to execute 5 - 2, to give an output of 2. How do we change the operator precedence?

You cannot really change the precedence, but you can add parentheses to group sub-expressions differently.

    >>> (5 - 2) * 2    6    >>> 5 - ( 2 * 2 )    1

Parentheses have the highest precedence in Python, and can be used to override operator precedence. (5 - 2) gets calculated first, and the final result of the expression is 6.

A positive thing about using parentheses is, that it makes expressions more readable. So even in situations such as 5 - 2 * 2, where we know the result according to precedence, adding parentheses is good.

Summary

In this step, we went about solving a few puzzles about expressions, touching concepts such as:

• SyntaxError for incorrect operators
• White-space in expressions
• Floating Point division by default
• Operator Precedence
• Using parentheses

Step 06: Printing Text

In the previous step, we learned how to use expressions to compute values. In this step, let's see how we can actually print multiplication table entries, that are readable by the user.

Snippet-01: Printing Text

How do we go about printing a complete multiplication table entry? We want to print text such as 5 * 6 = 30 . But trying to do so, as we know it, gives us a SyntaxError. Clearly, there is a different way to print text, as compared to an expression.

    >>> 5 * 6 = 30      File "<stdin>", line 1    SyntaxError: can't assign to operator

Let's first try to print a simple piece of text, Hello. Typing in this piece of code directly on Python Shell also gives us an error.

    >>> Hello    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'Hello' is not defined

Only expressions work that way, and Hello is not really an expression.

"Hello" is typically called a string, and represents the text of letters 'H', 'e', 'l', 'l', 'o'. "Hello" is hence different from the number 5.

There are a number of in-built functions in Python to help print strings. One of these is the print() function. Can you just say print Hello?

    >>> print Hello      File "<stdin>", line 1        print Hello                  ^    SyntaxError: Missing parentheses in call to 'print'. Did you mean print(Hello)?

The Python compiler gives you an error, that says "missing parentheses".

Will print(Hello) work?

    >>> print (Hello)    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'Hello' is not defined

Nope! Again, this one failed because you need to indicate that "Hello" is a string.

How do I indicate that "Hello" is a string? By putting it within double quotes.

Let's try print ("Hello")

    >>> print ("Hello")    Hello    >>> print("Hello")    Hello

print("Hello") finally results in "Hello" being printed out. To be able to print "Hello", the things we need to do are:

• Typing the method name print ,
• open parentheses ( ,
• Followed by a double quote " ,
• The text Hello,
• and another double quote " ,
• finished off with a closed parentheses ).

What we have written here is called a statement, a simple piece of code to execute. As part of this statement, we are calling a function, named print().

What exactly are we trying to print?

The text "Hello", which is called a parameter or argument, to print().

Now let's get back to what we wanted to do, which is to print 5 * 6 = 30. The most basic version would be something of this kind, print("5 * 6 = 30"). Here, we are passing the entire value in the form of a string.

    >>> print("5 * 6 = 30")    5 * 6 = 30

This prints the text on the console, as-is. The thing you need to understand here is, we aren't really calculating 30 using the formula 5 * 6, but directly putting text 30 in here. That's called hard-coding.

In a later step, we will look at how to actually calculate the value and pass it in.

Summary

In this step, we:

• Understood that displaying text on the console is not the same as printing an expression value
• Learned about the print() function, that is used to print text in Python.
• Found a way to print the text "5 * 6 = 30" on the console, by hard-coding values in a string

Step 07: Puzzles On Utility Methods, And Strings

In the previous step, we learned how to print 5 * 6 = 30. It was not a perfect solution, because we hard-coded everything. we used an in-built function named print(), passed a string to it, and invoked the method.

In this step, let's look at a number of puzzles related to in-built methods, their parameters, and strings in general.

For example, let's do print("5 * 6"), as in the previous step. What does this code result in?

    >>> print("5*6")    5*6    >>> print('5*6')    5*6

It just prints the string "5 * 6".

Let's say we try the code print(5 * 6),

    >>> print(5*6)    30

Without the double quotes, 5 * 6 is an expression. What will be the output? 30.

If you call print() with an expression argument, it prints the value of the expression. However, when we pass something within double quotes, it becomes a piece of text, printed as-is.

An interesting thing to note is, that in Python you can use either double-quotes (" and "), or single-quotes (' and ') with text values.

Let's look at a few other in-built methods within Python.

Consider abs() (which stands for absolute value), a method that accepts a numeric value. You can use abs(10.5), passing 10.5 as a value to it, and it prints the absolute value of 10.

    >>> abs 10.5      File "<stdin>", line 1        abs 10.5               ^    SyntaxError: invalid syntax    >>> abs(10.5)    10.5

If you pass in a string value, will it work? It complains, "abs() function will not work with a string, it only works with numeric values".

    >>> abs("10.5")    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    TypeError: bad operand type for abs(): 'str'

Let's say you want to use a function that computes "to the power of", for instance "2 to the power of 5". In Python, there's an in-built function named pow(), which does just what we need. To pow(), you can pass two parameters and calculate the result. How do you do that?

Will this work: pow 2 5? No, not at all. This code does not work as well: pow(2 5). pow(2, 5) is the correct syntax.

    >>> pow 2 5      File "<stdin>", line 1        pow 2 5            ^    SyntaxError: invalid syntax    >>> pow(2 5)      File "<stdin>", line 1        pow(2 5)              ^    SyntaxError: invalid syntax    >>> pow(2, 5)    32

You'll see that 32 is printed.

Let's see another example, "10 to the power of 3". pow(10,3) is the alternative to saying 10 ** 3. This gives us 1000, similar to how pow() would.

    >>> pow(10, 3)    1000    >>> 10 ** 3    1000

max() returns maximum in a set of numbers.min() function returns the minimum value.

    >>> max(34, 45, 67)    67    >>> min(34, 45, 67)    34

These are some of the in-built functions in Python, and we saw how to call the in-built functions by passing in a varied number of parameters.

Python is case sensitive. So let's say I want of calculate pow(2,5). So this would give me 32. Now, what if I say capital 'P' instead of small 'p' here? Pow(2,5) would lead to an error.

    >>> pow(2,5)    32    >>> Pow(2,5)    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'Pow' is not defined

The only things not case-sensitive in Python, are string values. Earlier we saw that the code print("Hello") displays the text "Hello". Inside a string, the text can be in any case. Hence, print("hello") displays "hello" ,with a small 'h'.

    >>> print("Hello")    Hello    >>> print("hello")    hello    >>> print("hellO")    hellO    >>> print ( "hellO" )    hellO

However inside your code, you need to be very particular about the case of function names, class names, variable names, and the like.

In your code, whitespace does not really matter. You can add space here and here, and you would still get the same output. However, in case of strings, whitespace does matter.

If we say print("hellO World"), it would print "hellO World", with a space in between. And if you do print("hellO World") with three spaces, it would print the same. In expressions, white-space does not affect the output.

    >>> print ( "hellO World" )    hellO World    >>> print ( "hellO     World" )    hellO     World

The last thing we want to look at, is an escape sequence. Let's say you want to print a double quote, ", in the code. If we were to do this: print("Hello""), what would happen? The compiler says error!

    >>> print("Hello"")      File "<stdin>", line 1        print("Hello"")                      ^    SyntaxError: EOL while scanning string literal

If you want to print a " inside a string, use an escape sequence. In Python, the symbol '\' is used as an escape character. On using '\' adjacent to the ", it prints Hello" (notice the trailing "). We have used the '\' to escape the ", by forming an escape sequence \".

>>> print("Hello\"")Hello">>>

The other reason why you would want to use a '\' is to print a <NEWLINE>. If you want to print "Hello World", but with "Hello" on one line and "World" on the next, '\n' is the escape sequence to use.

    >>> print("Hello\nWorld")    Hello    World

The other important escape sequence is '\t', which prints a <TAB> in the output. When you do print("Hello\tWorld"), you can see the tab-space between "Hello" and "World".

    >>> print("Hello\tWorld")    Hello   World

Another useful escape sequence is \\ . If you want to print a \ , then use the sequence \\ . You would see that it prints Hello\World . Think about what would happen if we put six \ . Yes you're right! It would print this string: "\\\" .

    >>> print("Hello\\World")    Hello\World    >>> print("Hello\\\\\\World")    Hello\\\World

One of the things with Python is, it does not matter whether you use double quotes or single quotes to enclose strings. There are some interesting, and useful ways of using a combination of both, within the same string. Have a look at this call: print("Hello'World"), and notice the output we get. In a similar way, the following code will be accepted and run by the Python system: print('Hello"World').

    >>> print('Hello"')    Hello"    >>> print("Hello'World")    Hello'World    >>> print("Hello\"World")    Hello"World    >>> print("Hello\"World")    Hello"World

The above two examples can be used as a tip by newbie programmers when they form string literals, and want to use them in their code:

• If the string literal contains one or more single quotes, then you can use double quotes to enclose it.
• However if the string contains one or more double quotes, then prefer to use single quotes to enclose it.

Summary

In this step, we:

• Explored a number of puzzles related to code involving:

  • Built-in functions for numeric calculations
  • The print() function to display expressions and strings

• Covered the following aspects of the above utilities:

  • Case-sensitive aspects of names and strings
  • The role played by whitespace
  • The escape character, and common escape sequences

Step 08: Formatted Output With print()

In the previous step, we learned how to print a hard-coded string, such as "5 * 6 = 30".

In this step, let's try to replace the hard-coded 30 with a computed value.

Let's start with a simple scenario. Let's say we want to place that calculated value within a string, and display it. How do we do that?

Snippet-01: print() Formatted Output

format() method can be used to print formatted text.

Let's see an example:

    >>> print("VALUE".format(5*2))    VALUE

We were expecting 10 to be printed, but it's actually printing VALUE.

How do we get 10 to be printed then?

    >>> print("VALUE {0}".format(5*2))    VALUE 10

By having an open brace {, closed brace }, and and by putting the index of the value between them. Here, the value is the first parameter, and it's index will be 0.

"VALUE {0}" is what we need.

Let's take another example. Suppose to the format() function, we pass three values: 10, 20 and 30.

Typically when we count positions or indexes, we start from 0.

To print the first value, you need to pass in an index of 0. To print the second value, pass an index of 1.

    >>> print("VALUE {0}".format(10,20,30))    VALUE 10    >>> print("VALUE {1}".format(10,20,30))    VALUE 20    >>> print("VALUE {2}".format(10,20,30))    VALUE 30

Now going back to our problem, we wanted to display "5 * 6 = 30", but without hard-coding. Instead of 30, we want the calculated value of 5 * 6.

    >>> print("5 * 6 = 30".format(5,6,5*6))    5 * 6 = 30

Let replace "5 * 6 = 30" with "5 * 6 = {2}". 2 is the index of parameter value 5*6.

    >>> print("5 * 6 = {2}".format(5,6,5*6))    5 * 6 = 30

Cool! Progress made.

Let's replace 5 * 6 with the right indices - {0} * {1}.

    >>> print("{0} * {1} = {2}".format(5,6,5*6))    5 * 6 = 30

The great thing about this, is now we can replace the values we passed to print() in the first place, without changing the indexes! So, we can display results for 5 * 7 = 35 and 5 * 8 = 40. We are now able to print 5 * 6 = 30, 5 * 7 = 35, 5 * 8 = 40, and can do similar things for other table entries as well.

    >>> print("{0} * {1} = {2}".format(5,7,5*7))    5 * 7 = 35    >>> print("{0} * {1} = {2}".format(5,8,5*8))    5 * 8 = 40    >>> print("{0} * {1} = {2}".format(5,8,5*8))    5 * 8 = 40

Summary

In this step, we:

• Discovered that Python provides a way to do formatted printing of string values
• Looked at the format() function, and saw how to call it within print()
• Observed how we could work only with the indexes of parameters to format(), and change the parameters we pass without changing the code

Step 09: Puzzles On format() and print()

In this step, let's look at a few puzzles related to the format, and the print methods.

Snippet-01: format() And print() Puzzles

Let's say we pass in additional values, such as: 5 * 8, 5 * 9 and 5 * 10. However, within the call to format(), we are only referring to the values at index 0, index 1 and index 2. The values at indexes 3 and 4 are not used at all. What would happen when we run the code?

    >>> print("{0} * {1} = {2}".format(5,8,5*8,5*9,5*10))    5 * 8 = 40

Would this throw an error? No, it does not. You can see that the additional values which are passed in, are conveniently ignored.

Let's say instead of passing in a value of 2, we pass 4. What would happen?

    >>> print("{0} * {1} = {4}".format(5,8,5*8,5*9,5*10))    5 * 8 = 50

5 * 10 is the value at index 4

Now let's take a different scenario. We remove all the parameters passed to format(). However, inside the call to print(), we continue to say {0} * {1} = {4}. So we are trying to print the value at index 4, but are only passing two values to the function format(). What do you think will happen?

    >>> print("{0} * {1} = {4}".format(5,8))    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    IndexError: tuple index out of range

It says IndexError, which means :"you are asking me to fetch the value at index 4, but only passing in two values. How can I do what you want?"

Let's look at a few more things related to other data types. We try to format the following inside print(): {0} * {1} = {2}, and would pass in 2.5, 2, and 2.5 * 2 . Here, 2 is an integer value, but 2.5 is a floating point value. You can see that it prints 2.5 * 2 = 5.0. So this approach of formatting values with print(), works also with floating point data as well.

    >>> print("{0} * {1} = {2}".format(2.5,2,2.5*2))    2.5 * 2 = 5.0

Now, are there are other types of data that format() works with? Yes, strings can join the party.

Let's say over here, we do: print("My name is {0}".format("Ranga")). What would happen?

    >>> print("My name is {0}".format("Ranga"))    My name is Ranga

Index 0 will be replaced with the first parameter to format().

Summary

In this step, we:

• Understood the behavior when the parameters passed to format():

  • Exceed the indexes accessed by print()
  • Are less than the indexes accessed by print()
  • Are of type integer, floating-point or string

Step 10: Introducing Variables

We are slowly making progress toward our main goal, which is to print the 5 multiplication table.

In the first statement, we are printing 5 * 1 = 5, and then changing the literals. To make it print 5 * 2 = 10, we are changing 1 to 2. Next, we are changing 2 to 3. How do we make it a little simpler, so that our effort is reduced?

    >>> print("{0} * {1} = {2}".format(5,1,5*1))    5 * 1 = 5    >>> print("{0} * {1} = {2}".format(5,2,5*2))    5 * 2 = 10    >>> print("{0} * {1} = {2}".format(5,3,5*3))    5 * 3 = 15

Let's try a different approach.

What would happen if you replace 1 with index, and 5 * 1 with 5 * index, and try to run it?

It gives an error! It says: "index is not defined".

Let's try and fix this, and execute index = 2. What would happen?

    >>> index = 2

Aha! This compiles.

    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 2 = 10

And this statement is printing 5 * 2 = 10.

Let's try something else. Let's make index = 3. What would happen?

    >>> index = 3    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 3 = 15

The same statement on being run, prints 5 * 3 = 15.

How can you check the value that index has? Just type in index.

    >>> index    3    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 3 = 15

The index symbol we have used here, is what is called a variable.

In Python, it's also called a name.

You can see that the value index referring to, can change over the duration of a program.

Initially, index was referring to a value of 1. later, index was referring to a value of 3.

Now, think about how you would print the entire table. All that you need to do, is start from 1, execute the same statement with print() and format(), to get output 5 * 1 = 5. Next, Change the value of index to 2, and then print the same statement. Next, index = 3, and print the same statement again.

    >>> index = 1    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 1 = 5    >>> index = 2    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 2 = 10    >>> index = 3    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 3 = 15

With the same statement print("{0} * {1} = {2}".format(5,index,5*index)), we are able to print different values. The value of index varies, but the code remains the same!

Variables make the program much more easier to read, as well as more generic.

Snippet-02: Classroom Exercise On Variables

Let's do a simple exercise with variables.

We want to create three variables a, b and c. Let's initially give them some values, say a value of 5 to a, 6 to b and 7 to c.

We want to get output of this kind: 5 + 6 + 7 = 18, without using the literal values.

You would want to use the values stored in the variables in a, b and c.

If you're hard-coding, the way to do it is with print("5 + 6 + 7 = 18").

    >>> a = 5    >>> b = 6    >>> c = 7    >>> print("5 + 6 + 7 = 18")    5 + 6 + 7 = 18    >>> print("5 + 6 + 7 = 18".format(a,b,c,a+b+c))    5 + 6 + 7 = 18

The way you can do that is with code like this: print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c)).

    >>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))    5 + 6 + 7 = 18

How do you confirm we are accessing values stored in the variables?

Let's change the values of a, b and c. Let's make a = 6 , b = 7 , and c = 8 . Execute same statement.

    >>> a = 6    >>> b = 7    >>> c = 8    >>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))    6 + 7 + 8 = 21

You can see the magic of variables at play here! Based on what values these variables are referring to, you can see that the output of the print statement changes.

Summary

In this step, we:

• Were introduced to variables, or names, in Python
• Observed how we could pass in values of variables to the format() function

Step 11: Puzzles On Variables

In the previous step, we were introduced to the concept of variables in Python.

We will start with looking at a few puzzles.

Snippet-01: Puzzles On Variables

What if I try to refer to a variable which is not yet created?

    >>> count    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'count' is not defined    >>> print(count)    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'count' is not defined

Before using a variable, you need to have it assigned a value. If you have not defined a variable before, then you cannot use it. Consider print(count), it does not know what count is. So it would throw an error, saying: "count is not defined, I have no idea what count is."

Once you assign a value to a variable, you can use it.

    >>> count = 4    >>> print(count)    4

The statement count = 4 where we are creating a variable named count for the first time, is called a variable definition.

This is the first time you're referring to a variable, and assigning a value to it.

Python will create a variable in its memory.

Variable names are case sensitive. count and Count are not the same thing.

    >>> Count    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'Count' is not defined    >>> count    4

There are rules to follow while naming variables.

All variable names should either start with an alphabet , or an underscore _ . count, _count are valid. 1count is invalid.

    >>> 1count = 5      File "<stdin>", line 1        1count = 5             ^    SyntaxError: invalid syntax    >>> count = 5    >>> _count = 5    >>> 1count      File "<stdin>", line 1        1count             ^    SyntaxError: invalid syntax    >>> 2count      File "<stdin>", line 1        2count             ^    SyntaxError: invalid syntax

After the first symbol, you can also use a numeral in variable names.

    >>> c12345 = 5

To summarize the rules for naming variables.

• This should start with an alphabet (a capital or a small alphabet) or underscore.
• Starting the second character, it can be alphabet, or underscore, or a numeric value.

Summary

In this step, we:

• Understood that a variable needs to be defined before it is used
• Learned that there are certain rules to be followed while giving names to variables

Step 12: Introducing Assignment

In this step, we will look at an important concept in Python, called assignment. In previous steps, we created variables, like i = 5.

Snippet-01: Introducing Assignment

You can create other variables using whatever value i is referring to. If we say j = i, what would happen?

    >>> i = 5    >>> j = i    >>> j    5

j would start referring to the same value that i is referring to. This statement is called an assignment.

Let's try j = 2 * i.

    >>> j = 2 * i    >>> j    10

j refers to a value of 10

= has a different meaning in programming compared to mathematics.

In mathematics, When we execute j = i, it means j and i are equal.

In prgramming, the value of the expression on right hand side is assigned to the variable on the right hand side. Can you use a constant on the left hand side of an assignment? The answer is "No"!

    >>> 5 = j      File "<stdin>", line 1    SyntaxError: can't assign to literal

The Python Shell throws an error, saying "Can't assign to literal", as 5 is a literal.

Let's create a couple of variables. num1 = 5 and num2 = 3. We would want to add these and create a fresh variable. Let's say the name of the variable is sum.

    >>> num1 = 5    >>> num2 = 3    >>> sum = num1 + num2    >>> sum    8

Create 3 variables a, b and c with different values and calculate their sum.

    >>> a = 5    >>> b = 6    >>> c = 7    >>> sum = a + b + c    >>> sum    18

We have just seen the mechanics of how assignment works in Python.

Summary

In this step, we:

• Learned what happens when you assign a value to a variable, which may or may not exist
• Discovered that literal constants cannot be placed on the left hand side of the assignment(=) operator

Step 13: Introducing Formatted Printing

Until now, we have been using the format() method to format and print values. Let's see a better approach to printing values.

This is the approach we used until now.

    >>> a = 1    >>> b = 2    >>> c = 3    >>> sum = a + b + c    >>> print("{0} + {1} + {2} = {3}".format(a, b, c ,sum))    1 + 2 + 3 = 6

Python has the concept of formatted strings. The syntax to use a formatted string is very simple - f"".

If we want to print the value of a variable a, we can use {a} in the text.

    >>> print(f"")    >>> print(f"value of a is {a}")    value of a is 1    >>> print(f"value of b is {b}")    value of b is 2

The variable within braces is replaced by its value.

You can use expressions in a formatted string. Example below uses {a+b}.

    >>> print(f"sum of a and b is {a + b}")    sum of a and b is 3

This feature was introduced in a Python 3 release.

Let's get back to the original problem we wanted to solve: printing 5 + 6 + 7 = 18, using formatted strings.

    >>> print(f"{a} + {b} + {c} = {sum}")    1 + 2 + 3 = 6

You can see how easy it turns out to be!

Step 14: The PMT-Challenge Revisited

We want to print the 5-table from 5 * 1 = 5 onward, until we reach to 5 * 10 = 50. The best solution we have right now, is shown below:

Snippet-01:

    >>> index = 1    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 1 = 5    >>> index = 2    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 2 = 10    >>> index = 3    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 3 = 15    >>> index = 4    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 4 = 20

Can we do something, to make sure that the code remains the same all the time, but the index value gets updated?

    >>> index = index + 1    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 5 = 25    >>> index = index + 1    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 6 = 30    >>> index = index + 1    >>> print("{0} * {1} = {2}".format(5,index,5*index))    5 * 7 = 35

We used index = index + 1 to increment index value.

If we execute these same two statements again and again, we can print the entire table! This is exactly what loops help us do: execute the same statements repeatedly.

The simplest loop available in Python is the for loop.

When we run a for loop, we need to specify the range of values - 1 to 10 or 1 to 20, and so on. range() function helps us to specify a range of values.

    >>> range(1,10)    range(1, 10)

The syntax of the for loop is: for i in range(1, 10): .... Here, i is the name of the control variable. In Python, you need to put a colon, ':', and in the next line give indentation.

    >>> for i in range(1,10):    ...   print(i)    ...    1    2    3    4    5    6    7    8    9

You would see that it prints from 1 to 9.

When we run a loop in range(1, 10), 1 is inclusive and 10 is exclusive.The loop runs from 1 to the value before 10, which is 9.

The leading whitespace before print(i) is called indentation. We'll talk about indentation later, when we talk about puzzles related to the for loop.

How can you extend this concept to solving our PMT-Challenge problem?

    >>> print(f"{5} * {index} = {5*index}")    5 * 7 = 35

What we were doing earlier, was calling print() with a formatted string. Now we want to print this statement for different values of i.

How can you do that?

Let's start with a simple example.

    >>> for i in range(1,11):    ...   print(f"{i}")    ...    1    2    3    4    5    6    7    8    9    10

print(f"{i}") prints the value of i.

Now, how do we get it to print 5 * 1 = 5 to 5 * 10 = 50?

    >>> for i in range(1,11):    ...   print(f"5 * {i} = {5 * i}")    ...    5 * 1 = 5    5 * 2 = 10    5 * 3 = 15    5 * 4 = 20    5 * 5 = 25    5 * 6 = 30    5 * 7 = 35    5 * 8 = 40    5 * 9 = 45    5 * 10 = 50    >>> 5 * 4 * 50    1000

print(f"5 * {i} = {5 * i}") prints a specific multiple of 5.

Step 15: Loops

In a previous step, we took a major step in programming. We wrote our first for loop with Python. In this step, let's try a few puzzles to understand the for loop even further.

The syntax of the for loop we looked at earlier was:

  for i in range(1, 10):    print(i)

Snippet-01:

Let's say we write a for loop, but don't give a : after the range() method, to close the first line. What would happen?

    >>> for i in range(1,10)      File "<stdin>", line 1        for i in range(1,10)                           ^        SyntaxError: invalid syntax

Invalid syntax. A : is mandatory within the for loop syntax.

Let's provide a : and in the next line, use print(i) without space before it (without indentation).

    >>> for i in range(1,10):    ... print(i)      File "<stdin>", line 2        print(i)            ^    IndentationError: expected an indented block

Most other programming languages use open brace { and closed brace } as delimiters in a for loop. However, Python uses indentation to identify which code is part of a for loop, and which is not. So if we are writing the body of a for loop, we must use indentation, and leave atleast a single <SPACE>.

    >>> for i in range(1,10):    ...   print(i)    ...    1    2    3    4    5    6    7    8    9

How do we execute two lines of code as part of the for loop?

    >>> for i in range(1,10):    ...  print(i)    ...  print(2*i)    ...    1    2    2    4    3    6    4    8    5    10    6    12    7    14    8    16    9    18

We are indenting both statements with a space - print(i) and print(2*i).

When for loop has only one line of code, you can specify it right after the :

    >>> for i in range(2,5): print(i)    ...    2    3    4

However, this is not considered to be a good programming practice. Even though you may want to execute just one statement in a for loop, indentation on a new line is recommended.

Another best practice is to use four <SPACE>s for indentation, instead of just two. This would give clear indentation of the code.

    >>> for i in range(2,5):    ...     print(i)    ...    2    3    4

Anybody who looks at the code immediately understands that this print() is part of the for loop.

Let's say you only want to print the odd numbers till 10, which are 1, 3, 5, 7 and 9. The range() function offers an interesting option.

    >>> for i in range (1,11,2):    ...   print(i)    ...    1    3    5    7    9

In for i in range(1, 11, 2), we pass in a third argument, called a step. After each iteration, the value of i is increment by step.

Summary

In this step, we:

• Looked at a few puzzles about the for loop, which lay emphasis on the following aspects of for:

  • The importance of syntax elements such as the colon
  • Indentation
  • Variations of the range() function

Step 16: Programming Exercise PE-BA-02

In the previous step, after initially exploring the Python for loop, we looked at a number of puzzles.

In this step, let's look at a few exercises.

Exercises

1. Print the even numbers up to 10. We would want to print 2 4 6 8 10, using a for loop.
2. Print the first 10 numbers in reverse
3. Print the first 10 even numbers in reverse
4. Print the squares of the first 10 numbers
5. Print the squares of the first 10 numbers, in reverse
6. Print the squares of the even numbers

Solution 1

Instead of starting with 1, we need to start with 2. Each time, i it would be incremented by 2, and 2 4 6 8 and 10 would be printed.

    >>> for i in range (2,11,2):    ...   print(i)    ...    2    4    6    8    10

Solution 2

We would want to print the numbers in reverse. Think about how you would do that using the range() function. We'd want go from 10, 9, 8, and so on up to 1.

    >>> for i in range (10,0,-1):    ...   print(i)    ...    10    9    8    7    6    5    4    3    2    1

The value to start with is 10. As we discussed earlier, the end value is exclusive. So to print from 10 to 1, we want to end one value which is 0. range(10, 0) seems to be what we need.

Usually these step value is positive, but we need to go backwards from 10. Hence, we would give a step value of -1.

Solution 3

Now, let's print the first 10 even numbers in reverse.

    >>> for i in range (20,0,-2):    ...   print(i)    ...    20    18    16    14    12    10    8    6    4    2

Solution 4

Next, we would want to print the squares of the first 10 numbers.

    >>> for i in range (1,11):    ...     print(i * i)    ...    1    4    9    16    25    36    49    64    81    100

Solution 5

Let's print the squares in the reverse order.

    >>> for i in range (10,0,-1):    ...     print(i*i)    ...    100    81    64    49    36    25    16    9    4    1

Solution 6

Print the squares of the even numbers. How to do that?

    >>> for i in range (10,0,-2):    ...     print(i*i)    ...    100    64    36    16    4

The key part is using a step of -2

We leave it as an exercise for you, to print squares of odd numbers.

Summary

In this video, we: * Tried out a few exercises involving the for loop, by playing around with printing sequences of numbers.

• Used the for loop to simplify the solution to the PMT-Challenge problem.

Step 17: Review: The Basics Of Python

It must have been a roller-coaster ride to solve the multiplication table challenge so far. If you're new to programming, there are a wide range of topics and concepts, that you would have learned during this small journey.

Let's quickly revise the important concepts we have learned during this small journey.

• 1, 11, 5, … are all called literals because these are constant values. Their values don't really change. _Consider 5 _4_ 50`. This is an expression. `_`is an operator, and`5`, `4`and`50 are operands.
• The name i in i = 1, is called a variable. It can refer to different values, at different points in time.
• range() and print() are in-built Python functions.
• Every complete line of code is called statement. The specific statement print(), is invoking a method. The other statement which we looked at earlier, was an assignment statement. index = index + 1 would evaluate index + 1, and have the index variable refer to that value.
• The syntax of the for loop was very simple. for var in range(1, 10) : ..., followed by statements you would want to execute in a loop, with indentation. For the sake of indentation we left four <SPACE>s in front of each statement inside the for loop.

So that, in a nutshell, is what we have learned over the course of our first section.

Chapter 03 - Introducing Methods

In the last section, we introduced you to the basics of python. We learned those concepts by applying them to solve the PMT-Challenge problem. The code below is what we ended up with as we solved that chellenge.

Snippet-01: Current Solution To PMT-Challenge

    >>> for i in range (1,11):    ...   print(f"8 * {i} = {8 * i}")

If we wanted to change the code to print the 7 table, we need to change the value 7 used in the for loop, to 8. It's simple, but still not as friendly as you would like.

    >>> for i in range (1,11):    ...   print(f"7 * {i} = {7 * i}")

To print a 7 table, it would be awesome if could say print_multiplication_table, and give a value of 7 beside it, and it would do the rest:

    >>> print_multiplication_table(7)    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'print_multiplication_table' is not defined    >>> print_multiplication_table(8)    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'print_multiplication_table' is not defined

Similarly, print_multiplication_table(8), could print the multiplication table for 8!

To be able to do this, we need to create a method, or a function. Creating a method makes the code reusable, and we can invoke that method very easily by passing arguments.

In this section, we take an in-depth look at methods.

Step 01: Defining Your First Method

Methods are very important building blocks in Python programming. In this step, we will create a simple method that prints "Hello World", twice.

Snippet-01:

When we talk about a method, we need to give it a name. We are already using an in-built Python method here, which is print().

    >>> print("Hello World")    Hello World    >>> print("Hello World")    Hello World

Similar to that, we need to give a name to our body of code. Let's say the name is print_hello_world_twice.

The syntax to create a method in Python is straightforward:

• At the start, use the keyword def followed by a space.
• Followed by name of the method - print_hello_world_twice.
• Add a pair of parenthesis: ().

• This is followed by a colon : (similar to what we used in a for loop).

  >>> def print_hello_world_twice():...     print("Hello World")...     print("Hello World")...

All statements in a method should be indented. The two print("Hello World") are indented. So, they are part of the method body.

print_hello_world_twice() defines a method, and it has certain code inside its body.

How do we call this method? Is it sufficient to say print_hello_world_twice?

    >>> print_hello_world_twice    <function print_hello_world_twice at 0x10a71ef28>

Python Shell says, there's a function defined with that specific name.

How do we execute a method? Very simple! Add a pair of parentheses to the name, ()!

    >>> print_hello_world_twice()    Hello World    Hello World    >>> print_hello_world_twice()    Hello World    Hello World

Now, we are able to run the method.

Summary

In this step, we:

• Learned we can define our own methods in the code we write
• Understood how to define a method, and all its syntax elements
• Saw how we can invoke a method we write

Step 02: Programming Exercise PE-MD-01

We will now leave you with two exercises, based on what we have learned about methods so far.

Exercises

1. Write a method called print_hello_world_thrice(). It should print "Hello World" thrice to the output. Define this method, and also invoke it.

2. Write and execute a method, that prints four statements:

   1. "I have created my first variable."
   2. "I've created in my first loop."
   3. "I've created my first method."
   4. "I am excited to learn Python." You need to print these four statements on four consecutive lines.

Solutions

Solution 1

    >>> def print_hello_world_thrice():    ...     print("Hello World")    ...     print("Hello World")    ...     print("Hello World")    ...    >>> print_hello_world_thrice()    Hello World    Hello World    Hello World

Solution 2

    >>> def print_your_progress():    ...     print("Statement 1")    ...     print("Statement 2")    ...     print("Statement 3")    ...     print("Statement 4")    ...    >>> print_your_progress()    Statement 1    Statement 2    Statement 3    Statement 4​    def print_your_progress():        print("Statement 1")        print("Statement 2")        print("Statement 3")        print("Statement 4")

For convenience, we have changed the exact text we need to print. Call this method with the syntax print_your_progress(), and you're able to execute its code.

Now try another exercise. We want to print "Statement 1", "Statement 2", "Statement 3" and "Statement 4" on different lines, using just one print statement. How can you do that?

    >>> def print_your_progress():    ...     print("Statement 1\nStatement 2\nStatement 3\nStatement 4")    ...    >>> print_your_progress()    Statement 1    Statement 2    Statement 3    Statement 4

We are using the newline character \n.

Let's look at the difference between defining and executing a method.

When we are writing a method definition, we are writing the code as part of its body. It has a specific syntax, and starts with the def keyword.

A definition by itself cannot cause the code in its body to be executed.

print_your_progress() represents a method call. The code inside the method is executed.

Summary

In this step, we:

• Implemented solutions to a few exercises that test our understanding of Python methods. We touched concepts such as:

  • Defining a method body
  • The way to invoke a method, to run its code
  • The difference between the two

Step 03: Passing Parameters To Methods

In the previous step,we created methods. We defined print_hello_world_twice(), and this printed "Hello World" twice. In this step, let's talk about method arguments, or parameters.

Snippet-01:

    >>> print_hello_world_twice()    Hello World    Hello World    >>> print_hello_world_thrice()    Hello World    Hello World    Hello World

Earlier, we wrote code for print_hello_world_thrice(), which prints the message three times.

Let's say you want to print it five times. You would need to write another method that does what you need. Doesn't that seem monotonous?

Instead of that, Won't it be great if I can call the method by the same name, say print_hello_world(5), and it would print "Hello World" five times?

The 5 which we are passing here is called an argument.

How do we define our method to accept this argument?

Let's call our argument no_of_times. If you have any experience with other programming languages, they generally need you to specify the parameter type. Something like This parameter is an integer/float/string, or other types. But Python does not require parameter type.

    >>> def print_hello_world(no_of_times):    ...    print("Hello World")    ...    print(no_of_times)    ...

Although we are not doing exactly what we set out to, let's see what would happen. What would happen if we say print_hello_world() ?

    >>> print_hello_world()    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    TypeError: print_hello_world() missing 1 required positional argument: 'no_of_times'

Error! Something like "Hey, you have created print_hello_world with a parameter, but not passing anything in here! Go ahead and pass a value". Let's pass in a value, such as 5.

    >>> print_hello_world(5)    Hello World    5    >>> print_hello_world(10)    Hello World    10    >>> print_hello_world(100)    Hello World    100

With print_hello_world(5), you can see "Hello World" and 5 being printed. We are now able to define this method to accept a value, and print that value by invoking it. You can pass in any value, such as10, 100, or others.

Now think of a different solution for this method, where you don't repeat the same piece of code to print "Hello World". Consider print_hello_world(5), it should still print "Hello World" 5 times. How do you do that?

Think about using something along the lines of a loop.

Snippet-02:

For now, what we are doing is we are printing "Hello World" 10 times.

    >>> def print_hello_world(no_of_times):    ...    for i in range(1,10):    ...       print("Hello World")    ...​    >>> print_hello_world(5)    Hello World    Hello World    Hello World    Hello World    Hello World    Hello World    Hello World    Hello World    Hello World

Our method call print_hello_world(5) now prints "Hello World" 10 times.

However just print the message 5 times. We need to make use of the parameter no_of_times inside the for loop as well.

    >>> def print_hello_world(no_of_times):    ...    for i in range(1,no_of_times):    ...       print("Hello World")    ...​    >>> print_hello_world(5)    Hello World    Hello World    Hello World    Hello World

Now let's execute the method again. You can see that it's printing 4 times only.

Why is it not printing 5 times?

That's because no_of_times as a second parameter to range() is exclusive.

    >>> def print_hello_world(no_of_times):    ...    for i in range(1,no_of_times+1):    ...       print("Hello World")    ...    >>> print_hello_world(5)    Hello World    Hello World    Hello World    Hello World    Hello World

Great, it's now printing the message 5 times!

    >>> print_hello_world(7)    Hello World    Hello World    Hello World    Hello World    Hello World    Hello World    Hello World

If you pass a different argument like 7, the message is displayed 7 times.

Something you need to always be cautious about in Python, is the indentation. Over here, the for loop is part of the method body. So we have extra indentation for it. The print is part of the for loop body. So guess what, even more indentation for that code.

Summary

In this step, we:

• Learned how to pass arguments to a method
• Understood that the method definition needs to have parameters coded in
• Observed that arguments passed during a method call can be accessed inside a methods body

Step 04: Classroom Exercise CE-MD-01

In this step, Let's look at a few exercises related to the method parameter.

Exercises

1. Write a method called print_numbers(), that would print all successive integers from 1 to n.
2. The second one is to write a method called print_squares_of_numbers(), that prints squares of all successive integers from 1 to n.

Solutions

Solution 1

    >>> def print_numbers(n):    ...    for i in range(1, n+1):    ...       print(i)    ...    >>> print_numbers(5)    1    2    3    4    5    >>>

If you are programming in other languages such as Java, you are used to naming methods in this way: printNumbers(). This convention is popularly known as "Camel Case".

That's NOT how Python programmers name their methods. Pythonic way is to use underscore _ to separate words in the method name, as in print_numbers().

Solution 2

Let's define print_squares_of_numbers(). This would be very similar to print_numbers(), working with the same range. Only, we need to say print(i*i) .

    >>> def print_squares_of_numbers(n):    ...    for i in range(1, n+1):    ...       print(i*i)    ...    >>> print_squares_of_numbers(5)    1    4    9    16    25

How is a parameter different from an argument?

• Inside the definition of the method, the name within parentheses is referred to as a parameter. In our recent exercise, n is a parameter, because it's used in the definition of print_squares_of_numbers.
• When you are passing a value to a method during a method call, say 5, that value is called an argument.

• Don't worry too much about it. Just follow this convention for now:

  • In the method call, call it an argument.
  • In a method definition, call it a parameter.

Summary

In this step, we looked at a few simple exercises related to passing method arguments

Step 05: Methods With Multiple Parameters

In this step, let's look at creating a method with multiple parameters.

Snippet-01:

print_hello_world accepts one parameter and prints "Hello World" the specified number of times.

    >>> def print_hello_world(no_of_times):    ...    for i in range(1,no_of_times+1):    ...       print("Hello World")    ...

Let's say we want to print another piece of text Welcome To Python, a specified number of times. How do you do that?

You can always create another method similar to the first one, such as print_welcome_to_python(no_of_times) and print the necessary text inside.

However, is that what a good programmer does?

A good programmer tries to create a more generic solution.

    >>> def print_string(str, no_of_times):    ...    for i in range(1,no_of_times+1):    ...       print(str)    ...    >>> print_string("Hello World", 3)    Hello World    Hello World    Hello World

The good programmer that you are, you created a new method called print_string(str, no_of_times) accepting a text parameter, in addition to no_of_times.

Syntax rules for method parameters are quite strict. If we say print_string("Welcome to Python") and run it, we get an error! Python Shell says: "I need no_of_times to be present in here".

    >>> print_string("Welcome to Python")    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    TypeError: print_string() missing 1 required positional argument: 'no_of_times'

Let's say you want to assign default values for str and no_of_times in print_string(). By default, we want to always print "Hello World", and that too 5 times.

The Python language makes this very easy. def print_string(str = "Hello World", no_of_times=5). The rest of the method remains the same.

    >>> def print_string(str="Hello World", no_of_times=5):    ...    for i in range(1,no_of_times+1):    ...       print(str)    ...

Now you can call print_string(), and "Hello World" is displayed 5 times.

    >>> print_string()    Hello World    Hello World    Hello World    Hello World    Hello World

If it's print_string("Welcome To Python"), what does it do? It prints "Welcome To Python", 5 times.

    >>> print_string("Welcome to Python")    Welcome to Python    Welcome to Python    Welcome to Python    Welcome to Python    Welcome to Python

Consider print_string("Welcome to Python", 8), it would print that string 8 times.

    >>> print_string("Welcome to Python", 8)    Welcome to Python    Welcome to Python    Welcome to Python    Welcome to Python    Welcome to Python    Welcome to Python    Welcome to Python    Welcome to Python

Isn't that cool!

Summary

In this step, we:

• Looked at how to pass multiple parameters to a method, starting with two arguments
• Learned how you can define default values for those parameters
• Observed we could pass default arguments for none, some or all of those parameters

Step 06: Back To Multiplication Table - Using Methods

Let's get back to our original goal, of why we needed methods. We wanted to create a multiplication table for a number, and observed that each time we needed to we needed change that number, we were forced to make a change in the code. This is not something we liked, and that's why we started investigating how methods can be used.

In this step, Let's try our hand at creating a multiplication table method.

Snippet-01:

    >>> for i in range (1,11):    ...    print(f"7 * {i} = {7 * i}")

Let's define a method called print_multiplication_table(), and pass in a parameter to it.

    >>> def print_multiplication_table(table):    ...    for i in range(1,11):    ...       print(f"{table} * {i} = {table * i}")    ...    >>> print_multiplication_table(7)    7 * 1 = 7    7 * 2 = 14    7 * 3 = 21    7 * 4 = 28    7 * 5 = 35    7 * 6 = 42    7 * 7 = 49    7 * 8 = 56    7 * 9 = 63    7 * 10 = 70

Now you have the entire multiplication table for 7.

You can then call print_multiplication_table() with arguments 8, 9,and so on, by simply changing the table arguemnt value.

We now want to create even better print_multiplication_table() method.

We want to control the start point, as well as the end point, in the call to range(). We want to say print_multiplication_table(7, 1, 6), to print the 7 table with entries from 1 to 6. How can you do that?

    >>> def print_multiplication_table(table, start, end):    ...    for i in range(start, end+1):    ...       print(f"{table} * {i} = {table * i}")    ...    >>> print_multiplication_table(7, 1 , 6)    7 * 1 = 7    7 * 2 = 14    7 * 3 = 21    7 * 4 = 28    7 * 5 = 35    7 * 6 = 42

Simple! Define those range limits as additional parameters!

The other thing we can obviously do, is have default values for the start, and the end.

    >>> def print_multiplication_table(table, start=1, end=10):    ...    for i in range(start, end+1):    ...       print(f"{table} * {i} = {table * i}")    ...​    >>> print_multiplication_table(7)    7 * 1 = 7    7 * 2 = 14    7 * 3 = 21    7 * 4 = 28    7 * 5 = 35    7 * 6 = 42    7 * 7 = 49    7 * 8 = 56    7 * 9 = 63    7 * 10 = 70

Calling print_multiplication_table(7) would give us entries from 7 * 1 = 7 to 7 * 10 = 70.

Now you can actually send out this method, to your friends, who would find it easy to use, and cool!

Summary

In this step, we:

• Learned how to define a method to print the multiplication table for a number
• Looked at how to enhance this method to make table printing more flexible
• Further enhanced that method to accept default arguments while printing a table

Step 07: Indentation Is King

In Python, indentation denote blocks of code. So if you want to put something in a for loop, or outside it, proper indentation would be sufficient. In this step, let's explore indentation in depth. Let's start by creating a simple method.

Snippet-01:

    >>> def method_to_understand_indentation():    ...     for i in range(1,11) :    ...        print(i)    ...    >>> method_to_understand_indentation()    1    2    3    4    5    6    7    8    9    10

Consider the code below: print(5) is indented at the same level as for loop.

    >>> def method_to_understand_indentation():    ...     for i in range(1,11) :    ...        print(i)    ...     print(5)    ...

You can see that print(5) is called only once. It is not part of the for loop.

    >>> method_to_understand_indentation()    1    2    3    4    5    6    7    8    9    10    5

Let's change the code in this method a bit. print(5) is indented the same way as print(i)

    >>> def method_to_understand_indentation():    ...     for i in range(1,11) :    ...        print(i)    ...        print(5)    ...

print(5) is part of the for loop. It is executed 10 times.

    >>> method_to_understand_indentation()    1    5    2    5    3    5    4    5    5    5    6    5    7    5    8    5    9    5    10    5

Whether we're talking about loops, methods or conditionals, proper indentation is very important in Python.

We indicate a block of code, by having all lines of that block at the same indentation level. There are no specific delimiters like for instance a pair of braces {...}, as in other programming languages.

Summary

In this step, we:

• Ran through a few examples to see how indentation works in Python

Step 08: Puzzles on Methods - Named Parameters

In this step, let's look at a variety of puzzles related to methods.

Snippet-01:

Consider the following method: I would want to print the default string 6 times. How do we do it?

    >>> def print_string(str="Hello World", no_of_times=5):    ...     for i in range(1,no_of_times+1):    ...        print(str)    ...    >>> print_string()    Hello World    Hello World    Hello World    Hello World    Hello World

Will it work if we call the method as in: print_string(6)?

    >>> print_string(6)    6    6    6    6    6

6 is passed as the first parameter. 6 is matched to str, and the method prints 6 the default number of times, which is 5.

to default to "Hello World", and print it 6 times.

You can do this in Python by using named parameters. During the method call, you can specify no_of_times = 6. no_of_times is a named parameter.

There is no provision of doing something like this, in other languages like Java.

Call it as print_string(no_of_times=6):

    >>> print_string(no_of_times=6)    Hello World    Hello World    Hello World    Hello World    Hello World    Hello World

str gets a default value, and "Hello World" is printed 6 times.

Named parameters are very useful, when a method has a number of parameters, and you would want to make it very clear which parameter you're passing a value for.

Let's call print_string(7, 8). what happens?

    >>> print_string(7, 8)    7    7    7    7    7    7    7    7

You would see that 7 is printed 8 times.

Since print() method is quite flexible, you can pass a number as the first argument. You can even pass a float.

    >>> print_string(7.5, 8)    7.5    7.5    7.5    7.5    7.5    7.5    7.5    7.5

What would be the result of this - print_string(7.5, "eight")?

    >>> print_string(7.5, "eight")    Traceback (most recent call last):      File "<stdin>", line 1, in <module>      File "<stdin>", line 2, in print_string    TypeError: must be str, not int

Note how no_of_times is used inside the method… as an argument to range(). range() only accepts integers, nothing else. When you run the code with print_string(7.5, "eight"), we get an error.

It says: TypeError: ```no_of_times``` must be ```int```, not string.

A simple rule of thumb is, if you have a parameter, you can pass any type of data to it. That could be an integer, a floating point value a string, or a boolean value. The Python language does not check for the type of a parameter. However, Python will throw an error if the function which is using that parameter, expects it to be of a specific type. The range() function expects that the no_of_times is an integer value.

Snippet-02:

The last thing which we would be looking at, is method naming conventions. We named our methods in a consistent way: print_string, print_multiplication_table, and the like.

This is exactly the format which most Python developers use, to name their methods.

Convention is to use underscore to separate words in a name.

However, there are a few rules for naming a method: One of the important rules is also related to variable names. We observed that a variable name cannot start with a number.

    >>> def 1_print():      File "<stdin>", line 1        def 1_print():             ^    SyntaxError: invalid token

Similarly, 1_print will not be accepted as a method name.

• You can start a name with an alphabet, or with an underscore.
• From the second character onward, you are allowed to use numeric symbols.

Methods and variables cannot be named using Python keywords.

Now, what is a keyword? For example, when we talked about for loop, as in:

```for i in range(1, 11): print(i)```...

• for is a keyword
• in is a keyword
• def is a keyword.

Later we will look at a few other keywords, such as while, return, if, else, elif, and many more.

    >>> def def():      File "<stdin>", line 1        def def():              ^    SyntaxError: invalid syntax    >>> def in():      File "<stdin>", line 1        def in():             ^    SyntaxError: invalid syntax    >>> def for():      File "<stdin>", line 1        def for():              ^    SyntaxError: invalid syntax

Summary

In this step, we:

• Were introduced to the concept of named parameters
• Explored the typical naming rules and conventions for methods in Python
• Observed that reserved keywords cannot be used to name variables or methods

Step 09: Methods - Return Values

Let's try and understand the importance of return values from a method. We will learn how to return a value from a method.

Snippet-01:

Let's name our method as product_of_two_numbers(), and let's have parameters a and b that it accepts:

    >>> def product_of_two_numbers(a,b):    ...     print(a * b)    ...    >>> product_of_two_numbers(1,2)    2

Can we take the product of these two numbers into a variable, and use it in other code, in the same program?

Suppose we say a product = product_of_two_numbers(1,2), is this allowed?

Let's run this code, and see what's stored in product.

    >>> product = product_of_two_numbers(1,2)    2    >>> product

It's empty.

The product_of_two_numbers() method is not really returning anything back, to be used elsewhere.

Have a look at some of the built-in Python functions, such as max() for example.

    >>> max(1,2,3)    3    >>> max(1,2,3,4)    4    >>> maximum = max(1,2,3,4)    >>> maximum    4    >>> maximum * 5    20

If I call max() with four parameters, as in maximum = max(1,2,3,4), the value 4 gets stored in maximum.

Later on in the code that follows, we can say maximum * 5, or we can print the value of maximum, or a similar calculation. This gives our programs a lot more flexibility.

So instead of just printing a*b, if this function could return a value, that would be quite useful.

    >>> def product_of_two_numbers(a,b):    ...      product = a * b;    ...      return product    ...    >>> product_of_two_numbers(2,3)    6

We are creating a variable product and doing a return product.

Lets run product_result = product_of_two_numbers(2, 3)

    >>> product_result = product_of_two_numbers(2,3)    >>> product_result    6    >>> product_result * 10    60

You can see how simple it is to return values from a method!

Summary

In this step, we:

• Learned how to return values from inside a method
• Observed how we can store the values returned by a method call

Step 10: Programming Exercise PE-MD-02

In this step let's look at a couple of exercises about returning values from methods.

Exercises

1. Write a method to return the sum of three integers.
2. Write a method which takes as input two integers, representing two angles of a triangle, and computes the third angle.

Hint: The sum of the angles in a triangle is 180 degrees. So if I am passing 50 and 50, 50 plus 50 is 100. So some of three angles should be 180, so the third angle will be 180 - 100, which is 80.

Solution 1

    >>>def sum_of_three_numbers(a, b, c):    ...     sum = a + b + c    ...     return sum    ...​    >>> sum_of_three_numbers(1,2,3)    6    >>> something = sum_of_three_numbers(1,2,3)    >>> something * 5    30

The shorter way of doing that would have been to have a temporary variable called instead of sum. We could directly return a + b + c.

    >>> def sum_of_three_numbers(a, b, c):    ...     return a + b + c    ...    >>> something = sum_of_three_numbers(1,2,3)    >>> something * 5    30

In methods, you can use return expression as well. That expression gets evaluated, and the value gets returned back. You'd see that the result remains the same.

Solution 2

The second is to write a method to take two integers, representing two angles of a triangle, and compute the third one.

    >>> def calculate_third_angle(first, second) :    ...     return 180 - ( first + second )    ...    >>> calculate_third_angle(50, 20)    110

In your programming career, you would be writing a number of methods. It's very important that you are comfortable doing so. Most of the methods that you write would return values back.

That's the reason why we're creating a lot of examples involving method calls.

Summary

In this step, we:

• Looked at a couple of exercises related to returning values from methods
• Observed that returning expressions avoids creating unnecessary variables, and shortens method definitions

Chapter 04 - Introduction To Python Platform

Until now we had been using Python Shell to execute all our code.

In the real world, we'll be write Python code in a variety of scripts. Before we would go into an IDE and use the IDE to write the script, we thought it would be useful for us to understand how you can write Python code without the benefit of an IDE.

This would also help us understand the Python environment, in-depth.

In the next few steps, we'll be looking at how to create simple Python scripts, using any text editor of your choice. Use Notepad, Notepad++. Editpad, or whichever text editing software you are comfortable with. We'll see what involved in executing the program, and what's happening in the background.

Here are a few videos you might want to look at.

• ​[Writing and Executing your First Python Script](https://www.youtube.com/watch?v=ORmDD1R7lNc)​
• ​[Understanding Python Virtual Machine and bytecode](https://www.youtube.com/watch?v=HE-FC0csG68)​

Step 01 - Writing and Executing Python Shell Programs

Here's a recommended video to watch - [Writing and Executing your First Python Script](https://www.youtube.com/watch?v=ORmDD1R7lNc)​

Let's get started with creating a simple script file.

We want to type in a simple Python script, or a piece of Python code, such as print("Hello world"). Does it get any simpler than this?

We'll save this into any folder on our hard disk, with a name 'first.py' .

first.py

print("Hello world")

The '.py' is not really mandatory, but typically all python files end with a '.py' extension.

Here's how you can run it:

• Launch your terminal, or command prompt
• 'cd' to the folder where this python script file is saved
• execute the command python first.py

You will see that Hello World will be printed.

If you are familiar with other programming languages, you would need a class, need to put the code in that class, and similar stuff.

While Python supports Object Oriented Programming, is not mandatory to create a class.

It's almost as if you're typing commands, starting from the line one! That's why we call it a python script.

Summary

In this small step, we tried to create a simple python script, and we ran it from the command line. All we needed to do, was use the same command we use to launch up the python shell, and followed it up with a name of the file. We created a file called first.py, executed that, and were able to see the output on the console.

As an exercise, try and add a few more methods and try to run those methods as well, as part of this script.

Step 02 - Python virtual machine and bytecode

In this step, let's try and understand what's happening in the background.

We wrote a simple piece of code using a text editor. We created a file named first.py, and all we did was: python3 first.py. If you look at other languages like Java for example, there is a separate compilation phase and then an execution phase. But with Python, just this command does both compilation and execution.

We saw that, as soon as we make a change and we run python3 first.py , the change is compiled and executed as well!

In Python, there is an intermediate format called Python byte code. Code is first compiled to bytecode, and then executed on the Python virtual machine.

When we installed Python, we installed both the python compiler and interpreter, as well as the virtual machine.

In Python, bytecode is not standardized. Different implementations of Python have different byte code. There are about 80 Python implementations, like CPython and Jython.

• CPython is a Python implementation in C language.
• Jython is a Python implementation in Java language. The bytecode which Jython uses is actually Java bytecode, and you can run it on the Java virtual machine.

Python leaves a lot of flexibility to the implementations of Python. They have the flexibility to choose the bytecode, and to choose the virtual machine that is compatible. The bytecode is tied to the specific virtual machine you are using. Therefore, if you're using CPython to compile the bytecode, you'll not be able to use Jython to run it.

You should make sure, that whatever implementation you are using to compile, is the same one you're using to run the code as well.

Summary

A lot of this sounds like boring theory. Don't worry about it. As a beginner, this might not be very important for you right now.

It's very important for you to understand the process. What's happening is you were writing Python code, and when you ran the command python3 first.py, it is both compiled and executed. An intermediate format called bytecode is created, which is not really standardized in Python. The bytecode is executed in a Python virtual machine.

The idea behind this quick section, is to give you a little bit of background on what's happening behind the scenes. I'll see you in the next section. Until then, bye-bye!

Chapter 05 - Introduction To VSCode

Let's start using the IDE VSCode to write our Python Code

Here are recommended videos to watch

• ​[Installing VSCode](https://www.youtube.com/watch?v=pI_cnCXpCTU)​
• ​[Write and Execute a Python File with VSCode](https://www.youtube.com/watch?v=Na05tSP21Jg)​
• ​[Write Your First Python Program with VSCode](https://www.youtube.com/watch?v=PvYSlWbXuCw)​

Step 01 - Installing and Introduction to VSCode

In this quick step, we'll help you install VSCode.

Here's the video guide for this step

• ​[Installing VSCode](https://www.youtube.com/watch?v=pI_cnCXpCTU)​

Go to Google and type in "VSCode Community Edition Download". Click the link which comes up first: [https://www.jetbrains.com/VSCode/download](https://www.jetbrains.com/VSCode/download).

You'll go to a page where you can choose the operating system: whether you are on Windows, Mac, or Linux.

Once you choose that, you can download the appropriate community version.

On the right hand side, you'll see a community version, and you can click the download link, to start the download.

If you are having a problem, you can also use the direct link to download.

Once you download VSCode, all you need to do is double-click the package which is downloaded. Follow the instructions, and you can continue with the defaults, until you completely install VSCode.

When you launch VSCode for the first time, it should ask you for a theme, where you can choose the default.

You're all set to go ahead with the next step in the course.

VSCode is an awesome IDE, and I'm sure you learn a lot about it.

Step 02 - Write and Execute a Python File with VSCode

In this step, let's launch up the VSCode IDE, and create our first Python project with a Python script. We want to be able to launch a Python script by the end of this step.

Here's the video guide for this step

• ​[Write and Execute a Python File with VSCode](https://www.youtube.com/watch?v=Na05tSP21Jg)​

Launch the VSCode IDE. You'll see that it takes a little while to launch the first time, and then brings up a welcome screen.

We would want to create a number of Python files. All these files will be in a project. You can think of our project as a collection of Python scripts, or modules.

To get started, let's create a new project by clicking 'create new project'. Let's name it - '01-first-python-project'.

Right now there are no files in the project.

Let's create our first Python file, using the IDE.

The way you can do that is by saying 'right-click' -> 'new' -> 'Python file', and then we'll give this a name of 'hello_world', and click OK.

Now you can go ahead and write your first Python program. Let's write some simple code, like print("Hello World"), and save it.

You can do a right-click here, and say 'Run hello_world'.

A small window comes up below, which shows the output. It says 'Hello World'.

Step 03 - Execise - Write Multiplication Table Method with VSCode

Let's start with a simple exercise. We created the multiplication table method in the Python Shell. What we do now, is we'll create the same thing but in a Python file of its own.

Here's the video guide for this step:

• ​[Write Your First Python Program with VSCode](https://www.youtube.com/watch?v=PvYSlWbXuCw)​

Chapter 06 - Introducing Data Types and Conditionals

Welcome to this section, where we will talk about numeric data types, and conditional program execution. After looking at the numeric and boolean data types, we will turn our attention to executing code, based on logical conditions.

Step 01: Numeric Data Types

In previous sections, we created variables of this kind: number = 5 , value = 2.5, etc. The 5 here is an integer, and integers represent numbers, such as 1, 2, 6, -1 and -2. In Python, the class for this particular data type is int.

If you write code like type(5), you'd get 'int' as the output.

In Python, there are no primitive types. What does that mean? Every value that you see in a Python program, is an object, an instance of some class.

In later sections, We'll understand what is a class, and what is an object or an instance. For now, the most important thing for you to remember, is that behind every value, there is a class.

Snippet-01:

Let's look at 2.5, which is a floating point value.

If you go ahead and do type(2.5), what would you see? You would see it's of type `float.

    >>> type(2.5)    <class 'float'>    >>> type(2.55)    <class 'float'>

When you perform a division operation between two integers, there is a chance that the result of the operation is a float. If you do 5/2, the result is 2.5. If we were to do 4/2, even then it's of type float.

    >>> type(5/2)    <class 'float'>    >>> type(4/2)    <class 'float'>    >>> 4/2    2.0    >>> 1 + 2    3

All the operations we looked at until now, can also be performed on floating point values.

    >>> value1 = 4.5    >>> value2 = 3.2    >>> value1 + value2    7.7    >>> value1 - value2    1.2999999999999998    >>> value1 / value2    1.40625    >>> value1 % value2    1.2999999999999998

value1 - value2 returns 1.299999999999998. Why?

Floating point numbers don't really represent accurate values. That's one of the things you need to always keep in mind.

Typically, if you're doing any highly sensitive financial calculations, don't use floats to represent your values. Instead, use Decimal. More about it later.

Operations can also be performed between int and float.

    >>> i + value1    14.5    >>> i - value1    5.5    >>> i / value1    2.2222222222222223    >>>

Result of an operation between a int and a float, is always a float.

Summary

In this step, we:

• Looked at the two basic numeric types: int and float.
• Saw the basic operations you can do among ints, among floats, and also between ints and floats.

Step 02: Programming Exercise PE-DT-01

In this step, let's do a simple exercise with numeric values.

Exercises

1. You need to create a method called simple_interest, and pass three parameters: principal, interest and duration (in years). You also want to calculate the amount after the specific duration, and return it back. Call this method with a few example values.

For example, if you want to call simple_interest with 10000, with an interest of 5 percent, for a duration of 5 years, the correct answer would be as follows: 10000 is the principal. In addition to 10000, you get the interest. The interest for one year is 10000 * 0.05, as the interest figure is in percentage.So that's 500 a year, into 5 which is 2500. The result would be 12500, and this value should be printed.

Solution 1

    def calculate_simple_interest(principal, interest, duration) :            return principal * (1 + interest * 0.01 * duration)​    print(calculate_simple_interest(10000,5,5))

Summary

In this step, we:

• Wrote a very simple method to do a simple interest calculation

Step 03: Puzzles On Numeric Types

In this section, we are looking at numeric types. In this specific step, we would be looking at a few puzzles related to values of these types.

Snippet-01:

Let's create a simple variable i = 1. i = i + 1. What would be the value of i after that?

    >>> i = 1    >>> i = i + 1    >>> i    2

It would be 2. There is a shortcut way of doing the same thing, by using the += operator.

    >>> i += 1    >>> i    3

Typically in other programming languages, you can do something of this kind: i++. There is no provision in Python to use increment operators like ++, in either prefix or suffix mode, like ++i, or i++.

    >>> i++      File "<stdin>", line 1        i++          ^    SyntaxError: invalid syntax    >>> ++i    3

Let's look at compound assignments.

    >>> i += 1    >>> i    4    >>> i -= 1    >>> i    3    >>> i /= 1    >>> i *= 2    >>> i    6.0

What you see here, is Dynamic Typing in Python. The type of a variable can change during the lifetime of the program.

>>> i = 2>>> type(i)<type 'int'>>>> i = i/2.0>>> type(i)<type 'float'>

Let's create a couple more numbers. number1 = 5 and number2 = 2. What could be the result of number1 / number2? You know it, it's 2.5 .

number1 // nummber2 truncates the value of 2.5, to 2.

    >>> number1//number2    2

If you can do number1 // number2, can you also do this: number1 //= number2?

    >>> number1 //= 2    >>> number1    2

5 ** 3 is 5 'to the power of' 3, which is 5 * 5 * 5, or 125.

    >>> 5 ** 3    125    >>> pow(5,3)    125

This can also be achieved by invoking pow(5, 3). We have an operator, as well as a method at our disposal.

The last thing we will look at, are type conversion functions.

If you need to convert an int value to a float, or a float to an int.

    >>> int(5.6)    5

What if you want to round a value? 5.6 is nearer to 6 than 5. You can use a function called round(), and here,round(5.6) gives the correct result 6.

    >>> round(5.6)    6    >>> round(5.4)    5    >>> round(5.5)    6

round() can also allows you to specify number of decimals in the result.

    >>> round(5.67, 1)    5.7    >>> round(5.678, 2)    5.68

You can also convert int to float, by using the function float().

    >>> float(5)    5.0

Summary

In this step, we:

• Looked at a few corner cases related to your numeric types.
• Examined the different operators available for use with values of numeric types
• Learned about the usage of type conversion functions

Step 04: Introducing Boolean Type

We will now shift our attention to the bool data type.

A boolean value is something which can be either "true" or "false".

Snippet-01:

In Python, "true" is represented by True, and "false" by False. It's important to remember that it's True with a capital 'T', and False with a capital 'F'.

    >>> True    True    >>> False    False    >>> true    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'true' is not defined    >>> false    Traceback (most recent call last):      File "<stdin>", line 1, in <module>    NameError: name 'false' is not defined

The boolean variable is_even indicates whether a number is even or not.

    >>> is_even = True    >>> is_odd = False

Let's create a variable i = 10. We want to find out if i > 15. What do you think is the result? False.

    >>> i = 10    >>> i > 15    False    >>> i < 15    True

In general, boolean values can represent the result of logical conditions.

Let's look at other operations that can result in bool values. We looked at > and <. Another operation which you can perform, is >=.

    >>> i >= 15    False    >>> i >= 10    True    >>> i > 10    False    >>> i <= 10    True    >>> i < 10    False

== is the comparison operator. We are only comparing the value of i against 10, not changing its value.

    >>> i == 10    True    >>> i == 11    False

Summary

In this step, we:

• Were introduced to the bool data type
• Learned that bool variables are useful handy while testing logical conditions

Step 05: Introducing Conditionals

In this step, let's look at if statement.

Sometimes you need to execute code only when certain conditions are true. You can use a if condition, which is the simplest conditional in Python. Let's look at an example.

Snippet-01:

Let's say i has a value of 5. You want to print something, only if i has a value greater than 3. How do you do that?

    >>> i = 5    >>> if i>3:    ...     print(f"{i} is greater than 3")    ...    5 is greater than 3

The syntax of the if is very simple: if followed by a condition; with the condition you want to check. It looks like: if i>3: ... You need to indent the body of the if with <SPACE>s as usual.

Let's say i has a value of 2. What would happen if we execute the same code again?

    >>> i = 2    >>> if i>3:    ...     print(f"{i} is greater than 3")    ...

You would see that nothing is printed to the console. Based on the value of i , either the statement is executed, or it's not. That's what an if helps us to do.

The way you can think about an if, is the body of code under the if is executed only when this condition is True. If this condition is not True, that code is not executed at all.

    >>> if(False):    ...   print("False")    ...    >>> if(True):    ...   print("True")    ...    True

Let's take two different numbers, say a = 5, and b = 7. We want to compare them, and predict if a is greater that b .

    >>> a = 5    >>> b = 7    >>> if(a>b):    ...   print("a is greater than b")    ...​    >>> a = 9    >>> if(a>b):    ...   print("a is greater than b")    ...    a is greater than b

Summary

In this step, we:

• Were introduced to the if statement, the simplest Python conditional
• Understood how an if helps in implementing conditional program logic

Step 06: Classroom Exercise CE-DT-01

In this step, let's look at a couple of exercises with the if statement.

Snippet-01:

Let's say we define four variables: a = 1, b = 2 , c = 3 and d = 5. we want to find out, if a + b is greater than c + d.

    >>> a = 1    >>> b = 2    >>> c = 3    >>> d = 5    >>> if a+b > c+d :    ...    print("a+b > c +d")    ...    >>> a = 9    >>> if a+b > c+d :    ...    print("a+b > c +d")    ...    a+b > c +d

Let's say we are given three values meant to be the angles of a triangle. Their values are angle1 = 30, angle2 = 20 and angle3 = 60. You want to find out if these three angles actually form a valid triangle. You know that the sum of the angles of a triangle is always 180 degrees.

    >>> angle1 = 30    >>> angle2 = 20    >>> angle3 = 60    >>> if(angle1 + angle2 + angle3 == 180):    ...      print("Valid Triangle")    ...    >>> angle2 = 90    >>> if(angle1 + angle2 + angle3 == 180):    ...      print("Valid Triangle")    ...    Valid Triangle

The last exercise is to check if a number is even or not.

Hint L you need to use one of the operators we talked about earlier. That's right, use the modulo operator %.

    >>> i = 2    >>> if(i%2==0):    ...   print("i is even")    ...    i is even​    >>> i = 3    >>> if(i%2==0):    ...   print("i is even")    ...

Summary

In this step, we:

• Looked at a few exercises related to the if statement, for writing and testing conditions.