What is a AssertionError?

An AssertionError is raised when an assert statement's condition evaluates to False. assert statements are meant for internal sanity checks during development, not for input validation in production code. They can be globally disabled by running Python with the -O flag.

Why it happens

• •An assert condition that fails because the program reached an unexpected state.
• •Using assert to validate user input or external data instead of raising a proper exception.
• •A test assertion failing, for example in pytest when the actual value does not match the expected value.
• •Running code that was written with assumptions that no longer hold after a refactor.

Examples

def divide(a, b):
    assert b != 0, "Divisor must not be zero"
    return a / b

print(divide(10, 0))

Traceback (most recent call last):
  File "app.py", line 5, in <module>
    print(divide(10, 0))
  File "app.py", line 2, in divide
    assert b != 0, "Divisor must not be zero"
AssertionError: Divisor must not be zero

def divide(a: float, b: float) -> float:
    if b == 0:
        raise ValueError("Divisor must not be zero")
    return a / b

try:
    print(divide(10, 0))
except ValueError as e:
    print(f"Error: {e}")

Related errors

• ValueError
• TypeError