all Python 3Exceptionsconstruct

assert

checking

An assertion — checks a condition and raises AssertionError when it is False. Used for debugging and tests. Disabled with -O.

Syntax

assert condition, 'message'

The classic trap is an extra pair of parentheses: assert (x > 0, 'message') tests a non-empty tuple, which is always truthy, so the check silently never fires (you only get a syntax warning). And because python -O strips assertions entirely, they must not guard user input, permissions or anything coming from outside — use a plain if with raise for that; assert is for invariants that cannot break unless the code itself is wrong.

Examples

def divide(a, b):
    assert b != 0, 'Denominator must not be zero'
    return a / b
print(divide(10, 2))  # → 5.0
try:
    divide(1, 0)
except AssertionError as e:
    print(e)  # → Denominator must not be zero

def get_positive(n):
    assert isinstance(n, int) and n > 0, f'Expected positive int, got {n!r}'
    return n
print(get_positive(5))  # → 5

def test_add():
    assert 1 + 1 == 2
    assert 'hello'.upper() == 'HELLO'
    assert len([1,2,3]) == 3
test_add()  # ничего не печатает — все assert верны

# assert с коллекцией
def first(lst):
    assert len(lst) > 0, 'List is empty'
    return lst[0]
print(first([10,20]))  # → 10

# ВАЖНО: assert отключается при python -O
# Не используй для валидации входных данных в продакшн!
x = 10
assert x > 0  # только для разработки

See also

Open in the full glossarydocs.python.orgReport an inaccuracy