Use assert to ensure that given condition is true.

assert True
assert 1 != 5
assert 1 in [1, 3]
v = 1
assert v == 2

The AssertionError error is raised if the condition is not true.

AssertionError(message="1 == 2 is not true")

Assertions are always compiled into test and debug binaries, but not by default into optimized application binaries.