Control Flow

if statement

Perhaps the most well-known statement type is if statement. For example:

if statement base example

>>> x = int(input("Please enter an integer: "))
>>> if x < 0:
...     x = 0
...     print("Negative x changed to 0")
... elif x == 0:
...     print("x is equal to zero")
... elif x == 1:
...     print("x is equal to one")
... else:
...     print("x is greater than one")

But let’s dive into if statement with more simple examples.

How it works

if statement is defined with a keyword if followed by a Boolean expression or any object and finished with colon. The statement requires a body: other statements to execute, also called an if block. The body is indented at the same distance from the left (in Python we use 4 spaces to indent a single block of code).

The body’s statements will be executed only in case if expression is True.

from random import randint

number: int = randint(1, 2)

if not number % 2:  # the as number % 2 == 0
    print(number, "is even")

The print statement on line #6 will be executed only for even value of number variable.

else block

if can be used by its own, it can extend its behavior with else block. The general syntax for else body is the same: at least one indented statement. But else doesn’t take any expression after it. This block of code will be executed only that the if statement truth check fails.

from random import randint

number: int = randint(1, 2)

if not number % 2:
    print(number, "is even")
else:
    print(number, "is odd")

You can attach only one else block to if statement. And you cannot use else statement on its own - this will cause SyntaxError.

elif block(s)

elif (else if) statement may be considered as semi-statement between if and else statements. In case your program has more than two dedicated choices you are able to extends control flow by appending elif blocks after if statement. The syntax is pretty similar to if statement. Each elif has its own boolean expression or an object to test for the truth value.

You can attach as many elif statements as it needed. But you cannot use elif without if statement.

Python will test conditions in if and elif statements from top to bottom. The first one, which considered to be True will be executed. All others will be skipped.

If there were no truth conditions else block will be executed (if exists).

>>> x = int(input("Enter some integer number: "))
>>> if not x % 5 and not x % 3: # the same as x % 5 == 0 and x % 3 == 0
...     print(x, "is divisible by 5 and 3")
... elif not x % 5:
...     print(x, "is divisible by 5")
... elif not x % 3:
...     print(x, "is divisible by 3")
... else:
...     print(x, "is not divisible by 5 or 3")

Note

The order conditions appears matter. The truth test goes from top to bottom and stops at first expression which is True.

Usage

if/elif/else statements help you to control which portion of your code is executed based on conditions from outer scope.

# Ask user for input
grade = int(input("Enter your grade (0-100): "))

# Use if/elif/else statements to assign letter grade
if grade >= 90:
    letter_grade = "A"
elif grade >= 80:
    letter_grade = "B"
elif grade >= 70:
    letter_grade = "C"
elif grade >= 60:
    letter_grade = "D"
else:
    letter_grade = "F"

# Print the letter grade
print("Your letter grade is:", letter_grade)