assert

Basically, assert could be a function:

def assert_(test, *args):
    if not test:
        raise AssertionError(*args)

assert_(2 + 2 == 4, 'the world is broken')

However, there are few advantages of assert as directive over assert as a function:

1. All asserts removed on the bytecode compilation step if optimization is enabled.

2. The message is lazy and executed only when needed:

assert False, print("executed")
# executed
# AssertionError: None

assert True, print("not executed")
# (prints nothing)