Python unittest successfully asserts None is False

Question

Why does assertFalse succeed on None?

import unittest

class TestNoneIsFalse(unittest.TestCase):
    def test_none_is_false(self):
        self.assertFalse(None)

Results:

> python -m unittest temp
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK

It seems as if this behaviour invites errors where a function does not always return a value. For example:

def is_lower_than_5(x):
    if x < 5:
        return True
    elif x > 5:
        return False

....

def test_5_is_not_lower_than_5(self):
   self.assertFalse(is_lower_than_5(5))

The above test would pass even though it should fail. It is missing an error in the code that should be caught.

How should we assert that the value is literally False and not merely false in a boolean context? e.g.

self.assertEquals(False, None)  # assert fails. good!

Answer 1

None is falsy, as well as 0, "", [], ...

assertFalse does not check whether the given value is False by identity. This behavior is consistent with the if statement:

if not None:
    print('falsy value!')

Similarly, assertTrue does not check whether a value is True, and as such values like 1, "abc", [1, 2, 3] pass the test. See Truth Value Testing for more information.

This behavior is also explicitly documented:

assertTrue(expr, msg=None)
assertFalse(expr, msg=None)

Test that expr is true (or false).

Note that this is equivalent to bool(expr) is True and not to expr is True

If you really want to be sure that a value is True or False, use assertIs.