Python 3 - Second Edition

20.2    Assertions

The assert keyword enables you to integrate assertions into a Python program. By writing an assert statement, the programmer specifies a condition that is essential for the execution of the program and that must always hold True when the assert statement is reached. If the condition of an assert statement evaluates to False, then an AssertionError exception is raised. In the following interactive mode session, several assert statements were entered:

>>> lst = [7, 1, 3, 5, -12]>>> assert max(lst) == 7>>> assert min(lst) == -12>>> assert sum(lst) == 0Traceback (most recent call last):  File "<python-input-3>", line 1, in <module>    assert sum(lst) == 0           ^^^^^^^^^^^^^AssertionError  

An error message can also be specified in the ...