Assert statement in Python

The assert statement is a debugging and testing tool used to check conditions in your code.

• If the condition is True → the program continues normally.
• If the condition is False → Python raises AssertionError.

In simple terms: “Assert that this condition must be true.”

Basic Syntax

assert condition

# With custom error message
assert condition, "Error message if condition fails"    

How assert Works (Concept)

Python

assert x > 0    

This means Python checks if x > 0. If the condition is false, Python stops execution and raises an error.

Equivalent logic:

Python

if not x > 0:
    raise AssertionError    

Example: Basic Assertion

Python

x = 10

assert x > 0  # This will pass (no error)
print("x is positive")    

Example: Assertion Failure

Python

x = -5

assert x > 0  # This will fail
print("This line will not execute")    

Output: AssertionError

Example: Custom Error Message

Python

age = 15

assert age >= 18, "User must be at least 18 years old"    

Output: AssertionError: User must be at least 18 years old

Example: assert in Functions

Python

def divide(a, b):
    # Ensure denominator is not zero
    assert b != 0, "Denominator cannot be zero"
    return a / b

print(divide(10, 2))  # Works
print(divide(10, 0))  # AssertionError