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Incorrect instantiation of incomplete ABC subclass in test_collections::validate_abstract_methods` #137463

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Incorrect instantiation of incomplete ABC subclass in test_collections::validate_abstract_methods`#137463
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stdlibPython modules in the Lib dirtestsTests in the Lib/test dirtype-bugAn unexpected behavior, bug, or error
@guilhermeleobas

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@guilhermeleobas
guilhermeleobas
opened on Aug 6, 2025

Bug report

Bug description:

The test for missing abstract methods in validate_abstract_methods incorrectly attempted to instantiate the generated class C with an argument (C(name)), which always raises a TypeError: C() takes no arguments. Although the test originally passes, it passes for the wrong reason.

cpython/Lib/test/test_collections.py

Lines 727 to 739 in 481d5b5

def validate_abstract_methods(self, abc, *names):
methodstubs = dict.fromkeys(names, lambda s, *args: 0)
# everything should work will all required methods are present
C = type('C', (abc,), methodstubs)
C()
# instantiation should fail if a required method is missing
for name in names:
stubs = methodstubs.copy()
del stubs[name]
C = type('C', (abc,), stubs)
self.assertRaises(TypeError, C, name)

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

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    stdlibPython modules in the Lib dirtestsTests in the Lib/test dirtype-bugAn unexpected behavior, bug, or error

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