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Using ``assert`` for testing
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The Python ``assert`` statement means - "raise an error unles the following
expression is equivalent to True".

By "equivalent to True", I mean the expression returns True from Python
:doc:`truth value testing <truthiness>`.

``assert`` raises an ``AssertionError`` if the statement is equivalent to
False.  It does nothing if the statement is equivalent to True.

So, if you ``assert an_expression`` and there is no error, then the result of
``an_expression`` was equivalent to True.  The following expressions evaluate
to True, and therefore the asserts do not raise an error:

>>> assert True
>>> assert 10 == 10
>>> assert 10 % 2 == 0

These expressions are equivalent to False, and the asserts do raise errors:

>>> assert False
Traceback (most recent call last):
   ...
AssertionError
>>> assert 10 == 11
Traceback (most recent call last):
   ...
AssertionError

Although ``assert`` does work with expression values of True and False, the test
that assert uses is more general than ``expr_result == True``.  In fact,
assert uses :doc:`truth value testing <truthiness>` to decide whether to raise
an ``AssertionError`` or not:

>>> assert ['some', 'elements']  # not-empty list tests as True
>>> assert []  # an empty list tests as False
Traceback (most recent call last):
   ...
AssertionError
>>> assert 10  # any number other than zero evaluates as True
>>> assert 1
>>> assert 0
Traceback (most recent call last):
   ...
AssertionError
>>> assert None
Traceback (most recent call last):
   ...
AssertionError