Python If Statement - Syntax, Flow Diagram, Examples

Python IF

Python If statement is a conditional statement wherein a set of statements execute based on the result of a condition.

https://youtu.be/hkzm_L_4GSE?si=ipp2xYwxQsu9dKC8

In this tutorial, you'll learn about Python If statement, its syntax, and different scenarios where Python If statement can be used.

Execution Flow Diagram

Following is a flow diagram of Python if statement. Based on the evaluation of condition, program execution takes one of the paths.

Syntax of If Statement

Following is the syntax of if-statement in Python.

if boolean_expression:
    statement(s)

Observe the indentation provided for statement(s) inside if block and the colon : after boolean expression.

If the boolean expression returns true, the statement(s) of the if block are executed. Else, the statement(s) are not executed, and the program execution continues with the statements after if statement, if there are any.

Following is a flowchart depicting the execution of condition and statements in the if statement.

Examples

1. Simple example for If statement

In this example, we will use a simple boolean expression formed with relational operator, less than, for the if statement condition. The statements(s) inside the if block is just a single print statement.

Python Program

a = 2
b = 5

if a<b:
    print(a, 'is less than', b)

Output

Run the program, and you will see the following output in console.

2 is less than 5

It is trivial that the condition provided in the above if statement evaluates to true, therefore the statement(s) inside the if block is executed.

2. Python If Statement where Boolean Expression is False

In this example, we will write an if statement where the boolean expression or condition evaluates to false.

Python Program

a = 24
b = 5

if a<b:
    print(a, 'is less than', b)

No Output

The condition provided in the If statement evaluates to False, and therefore the statement inside the if-block is not executed.

3. If statement with compound condition

When you need to join multiple conditions in a single boolean expression, use logical operators to join them and create a compound condition as shown in the following program. The logical operators could be: python and, python or or python not.

Python Program

a = 2
b = 5
c = 4

if a<b and a<c:
   print(a, 'is less than', b, 'and', c)

Output

2 is less than 5 and 4

a<b is true and a<c is true. As a result the whole condition is True and if-block is executed.

4. If statement with condition evaluating to a number

If the expression in the If statement evaluates to a number, then the statement(s) in if-block are executed if the number is non-zero. zero is considered to be False and non-zero (positive or negative) is considered True.

In this example, we write a Python If statement, where the boolean expression evaluates to a number.

Python Program

a = 2

if a:
    print(a, 'is not zero')

Output

2 is not zero.

As the value of condition evaluates to 2, which is a non-zero number, the statement(s) inside if block are executed.

5. If statement with multiple statements in if-block

In the syntax section, we already mentioned that there can be multiple statements inside if block.

In this example, we write three statements inside if block. Please note that these statements must have same indentation.

Python Program

a = 10

if a > 2 :
    print(a, 'is not zero')
    print('And this is another statement')
    print('Yet another statement')

We have written only print statements in this example inside if block. But you may write a set of any valid python statement(s) inside the if block.

Please note that the statements of if-block have same indentation.

Output

2 is not zero
And this is another statement
Yet another statement

6. Nested If statement

You can write a Python If statement inside another Python If statement. This is called nesting.

In this example, we will write a Nested If statement: an If statement inside another If statement.

Python Program

a = 2

if a!=0:
    print(a, 'is not zero.')
    if a>0:
        print(a, 'is positive.')
    if a<0:
        print(a, 'is negative.')

Output

2 is not zero.
2 is positive.

As the first/outer condition a!=0 evaluates to true, the execution enters the if block. And the if statements inside this outer if block are considered as yet another Python statements and executed accordingly.

Summary

In this tutorial of Python Examples, we learned what an If statement is, its syntax, its usage, how to write nested If statement.