Python assert() function

The assert statement is a debugging tool in Python that checks a condition. If the condition evaluates to False, it raises an error with an optional custom message, helping to identify issues in the code during development.

Example

x = 10

assert x > 5  # Passes, as x > 5

assert x < 5, "x must be less than 5"  # Fails, raises AssertionError with message

Syntax

assert condition, optional_message

condition: The expression to evaluate. If True, the program continues; if False, it raises AssertionError.
optional_message: A custom error message displayed when the assertion fails.

Why Use assert?

The assert statement is perfect for developers to quickly validate assumptions about their code, catch unexpected bugs, and ensure the program behaves as expected during the development phase. It’s a simple way to document and enforce rules that your code relies on.

1. Validating User Input

Scenario: A program accepts user input for age. You want to ensure the input is non-negative.

user_age = -5

assert user_age >= 0, "Age cannot be negative"

If user_age is negative, the program raises an AssertionError with the message:
AssertionError: Age cannot be negative

2. Checking Function Output

Scenario: A function calculates the area of a rectangle. You want to ensure the calculated area is valid.

def calculate_area(length, width):

    return length * width

area = calculate_area(-3, 5)

assert area > 0, "Area calculation failed, negative result"

If the calculated area is negative, the program halts with the error message:

AssertionError: Area calculation failed, negative result

3. Ensuring Non-Empty Data

Scenario: A data processing task requires a non-empty list. You want to verify that the input list contains data.

data = []

assert data, "Data list cannot be empty"

If data is an empty list, the program raises an AssertionError with the message:
AssertionError: Data list cannot be empty

Key Notes

Use During Development: Assertions are best suited for debugging and catching issues early during development.
Avoid in Production: Assertions can be globally disabled in optimized mode (-O flag), making them unsuitable for critical runtime checks.
Helpful Messages: Include informative error messages to make debugging easier.
Do Not Overuse: Assertions should highlight assumptions and potential bugs, not replace validation logic or exception handling.

By integrating assert statements effectively, you can create robust code that quickly catches unexpected conditions during development.