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bpo-41877 Check for asert, aseert, assrt in mocks (GH-23165)
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Currently, a Mock object which is not unsafe will raise an
AttributeError if an attribute with the prefix assert or assret is
accessed on it. This protects against misspellings of real assert
method calls, which lead to tests passing silently even if the tested
code does not satisfy the intended assertion.

Recently a check was done in a large code base (Google) and three
more frequent ways of misspelling assert were found causing harm:
asert, aseert, assrt. These are now added to the existing check.
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vabr-g committed Nov 5, 2020
1 parent 133aa2d commit 4662fa9
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Showing 3 changed files with 14 additions and 3 deletions.
4 changes: 2 additions & 2 deletions Lib/unittest/
Expand Up @@ -631,9 +631,9 @@ def __getattr__(self, name):
elif _is_magic(name):
raise AttributeError(name)
if not self._mock_unsafe:
if name.startswith(('assert', 'assret')):
if name.startswith(('assert', 'assret', 'asert', 'aseert', 'assrt')):
raise AttributeError("Attributes cannot start with 'assert' "
"or 'assret'")
"or its misspellings")

result = self._mock_children.get(name)
if result is _deleted:
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11 changes: 10 additions & 1 deletion Lib/unittest/test/testmock/
Expand Up @@ -1598,14 +1598,23 @@ def static_method(): pass
def test_mock_unsafe(self):
m = Mock()
msg = "Attributes cannot start with 'assert' or 'assret'"
msg = "Attributes cannot start with 'assert' or its misspellings"
with self.assertRaisesRegex(AttributeError, msg):
with self.assertRaisesRegex(AttributeError, msg):
with self.assertRaisesRegex(AttributeError, msg):
with self.assertRaisesRegex(AttributeError, msg):
with self.assertRaisesRegex(AttributeError, msg):
m = Mock(unsafe=True)

def test_assert_not_called(self):
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@@ -0,0 +1,2 @@
Mock objects which are not unsafe will now raise an AttributeError if an attribute with the prefix asert, aseert,
or assrt is accessed, in addition to this already happening for the prefixes assert or assret.

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