Why You Should Avoid Using assert for Validation in Python

Posted by Afsal on 21-Jun-2024

In Python programming, using assert statements can be tempting for checking conditions and validating data. However, using assert for validation, especially in production code, is considered a bad practice. In this post, we’ll explore why you should avoid using assert for validation and what alternatives you can use instead.

What is assert?

The assert statement is used for debugging purposes. It tests a condition and triggers an AssertionError if the condition is false. Here’s a simple example:

def divide(a, b):
    assert b != 0, "Denominator cannot be zero"
    return a / b

In this example, the assert statement ensures that the denominator is not zero before performing the division.

Why assert Should Not Be Used for Validation

Optimization Mode (-O): When Python is run with optimization flags (python -O or python -OO), all assert statements are stripped out and ignored. This means that any validation relying on assert will be bypassed, potentially leading to undetected errors and unpredictable behavior in your program.

Non-Standard Error Handling:  Assert raises an AssertionError which is generally used for internal errors and debugging, not for handling user errors or input validation. Proper validation should raise more specific exceptions like ValueError, TypeError, etc., which are more informative and appropriate for error handling in production.

Readability and Intent: Assertions are meant for checking internal correctness and invariants during development. Using them for input validation conflates their purpose, making the code harder to understand and maintain. Explicitly using condition checks and raising appropriate exceptions makes the code clearer and expresses the programmer's intent more effectively.

Instead of using assert, perform explicit checks and raise suitable exceptions. This ensures that validation is always enforced, regardless of how the Python interpreter is run.

While assert statements are useful for debugging and ensuring internal correctness during development, they are not suitable for validating user inputs or critical data in production code. By using explicit checks, raising appropriate exceptions, and leveraging validation libraries, you can write more robust, clear, and maintainable Python code. Avoiding assert for validation helps ensure that your program behaves correctly and predictably in all environments.