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Python if Statements

Python Conditional Statements​

Almost in every program it is necessary to execute code only under certain conditions.

To be able to do this, the following conditional statements are available in Python:

  • if
  • if..else
  • Nested if
  • if-elif statements
note

Python uses the if keyword to implement decision control. It is fundamental for every conditional statements.

note

The conditions to be tested in conditional statements are expressions that are evaluated in Boolean value. Therefore, they respect boolean datatypes.

In addition, Python interprets nonzero values as True. None and 0 are interpreted as False.

warning

Indentation is used to delimit the block of code.

Failure or incorrect indentation may result in errors!

Python if Statement​

The if statement is the simplest conditional instruction: it executes inner block of instructions only if the condition is evaluated True.

if condition:
    # statements to execute 
    # if condition is true

For example:

a = 10
if a > 15:
    print(f"{a} is greater than 15")

a = -1
if a > 15:
    print(f"{a} is greater than 15")
    
print("The end")

Output

10 is greater than 15
The end

Python if-else Statement​

The if..else statement evaluates a condition and executes the inner instruction block of if only if the test condition is True. Otherwise, if the condition is False, the inner instruction block of else is executed.

if condition:
    # statements to execute 
    # if condition is true
else:
    # statements to execute
    # if condition is false

For example:

a = 10
if a % 2 == 0:
    print("Even number")
else:
    print("Odd number")

Python if-elif-else Statement​

elif stands for else if: it allows you to check for multiple expressions.

if condition:
    # statements to execute 
elif condition:
    # statements to execute
..
..
..
else:
   # statements to execute
  • If the condition of if is False, check the condition of the next block elif and so on.
  • If all the conditions are False, the inner instruction block of else is executed.
note

Only one block among several if...elif...else blocks is executed according to the condition.

warning

The if block can have only one else block. But it can have multiple elif blocks.

For example:

num = 1.2
# You can also try with these:
# num = 0
# num = -1.1

if num > 0:
    print("Positive number")
elif num == 0:
    print("Zero")
else:
    print("Negative number")

where:

  • if num is positive, Positive number is printed.
  • If num is equal to 0, Zero is printed.
  • If num is negative, Negative number is printed.

Python Nested If​

if...elif...else statement can be nested inside another if...elif...else statement.

note

Any number of these statements can be nested within each other.

if condition1:
    # statements to execute 
    # if condition1 is true
    if condition2:
        # statements to execute 
        # if condition2 is true
    # end nested if block
# end outer if block

For example:

num = int(input("Enter a number: "))
if num >= 0:
    if num == 0:
        print("Zero")
    else:
        print("Positive number")
else:
    print("Negative number")

And some output with different numbers:

Enter a number: 10
Positive number
Enter a number: 0
Zero
Enter a number: -5
Negative number

Shorthand if and if-else​

Shorthand if and if-else are if statements that are written on the same line as the if statement.

They are known as Conditional Expressions (or Ternary Operators) and are discussed in detail in the dedicated chapter.

if a > b: print("a is greater than b")

a = 1
b = 10
print("a") if a > b else print("b")

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