AssertionError

AssertionError

Traceback

terminal

Traceback (most recent call last):
  File "main.py", line 5, in <module>
    set_age(-5)
AssertionError

What causes this error

An assert statement evaluated to False. This indicates a bug or violated assumption in the code's logic.

How to fix it

Fix the code so the asserted condition is true, or fix the assert if the condition was wrong. Never use assert for user input validation — use explicit if/raise instead. Remember assertions are removed with `python -O`.

Code that causes this error

Broken

def set_age(age):
    assert age > 0  # fails for negative input
    return age

set_age(-5)

Fixed code

Fixed

def set_age(age):
    if age <= 0:
        raise ValueError(f"Age must be positive, got {age}")
    return age

try:
    set_age(-5)
except ValueError as e:
    print(e)

About AssertionError

An AssertionError is raised when an `assert` statement fails. The assert statement is used to verify conditions that should always be true at a particular point in the program — if the condition is False, it raises AssertionError with an optional message. Assertions are a debugging and development tool, not a control flow mechanism.

They can be globally disabled by running Python with the `-O` (optimize) flag, which strips all assert statements from the bytecode. For this reason, assert should never be used for input validation, access control, or any check that must always run in production. Use assert for internal invariants, preconditions in development, and self-tests.

The unittest framework uses assertion methods (assertEqual, assertTrue, etc.) that raise AssertionError with detailed failure messages, making it the backbone of Python testing.

Common scenarios

Assert statements failing due to incorrect assumptions in code logic

Using assertions for input validation instead of explicit checks

Boolean conditions that evaluate differently than expected

Off-by-one errors and boundary condition mistakes in algorithms