In Python, assertions are not executed when optimizations are enabled. This may lead to unexpected behavior when assertions are used to the check validity of a piece of input.

If the value being tested is false, replace the assert statement with a raise statement that raises an appropriate exception. If the value being tested is true, delete the assert statement or replace it with a pass statement.

This example shows a function buy_bananas that takes a number n as input. The function checks that this number is not too big before sending off an order for that number of bananas. Because this is done using an assert statement, the check disappears when optimizations are enabled. The second function corrects this error by explicitly raising an AssertionError, and checks the value even when optimizations are enabled.

Python Language Reference: The assert statement.

The Python Tutorial: “Compiled” Python files.