unittest.mock spec calls class properties

[melwitt]

BPO	41768
Nosy	@terryjreedy, @cjw296, @voidspace, @ambv, @lisroach, @mariocj89, @tirkarthi, @sobolevn, @melwitt
PRs	gh-85934: Use getattr_static when adding mock spec #22209

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-09-12.01:10:24.232>
labels = ['3.8', 'type-bug', 'library', '3.9', '3.10']
title = 'unittest.mock spec calls class properties'
updated_at = <Date 2021-12-03.08:45:33.635>
user = 'https://github.com/melwitt'

bugs.python.org fields:

activity = <Date 2021-12-03.08:45:33.635>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-09-12.01:10:24.232>
creator = 'melwitt'
dependencies = []
files = []
hgrepos = []
issue_num = 41768
keywords = ['patch']
message_count = 3.0
messages = ['376757', '377140', '407569']
nosy_count = 9.0
nosy_names = ['terry.reedy', 'cjw296', 'michael.foord', 'lukasz.langa', 'lisroach', 'mariocj89', 'xtreak', 'sobolevn', 'melwitt']
pr_nums = ['22209']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41768'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[melwitt]

When async magic method support was added to unittest.mock.Mock to address issue bpo-26467, it introduced a getattr call [1] that causes class properties to be called when the class is used as a mock spec.

This caused a problem for a test in my project when running with Python 3.8 where previously the test worked OK with Python 3.6.

The test aims to verify that a class instance is not created if the called code path does not access the class property and thus the class will not create a heavy object unless it's needed (lazy create on access via @Property).

As of Python 3.8, the @Property is always called and is called by the mock spec process itself, even though the code path being tested does not access the class @Property.

Here is a code snippet that illustrates the @Property calling from the mock spec alone:

class SomethingElse(object):
    def __init__(self):
        self._instance = None

    @property
    def instance(self):
        if not self._instance:
            self._instance = 'object'

...

    def test_property_not_called_with_spec_mock(self):
        obj = SomethingElse()
        self.assertIsNone(obj._instance)
        mock = Mock(spec=obj)
        self.assertIsNone(obj._instance)

$ ./python -m unittest -v unittest.test.testmock.testmock.MockTest.test_property_not_called_with_spec_mock
test_property_not_called_with_spec_mock (unittest.test.testmock.testmock.MockTest) ... FAIL


======================================================================
FAIL: test_property_not_called_with_spec_mock (unittest.test.testmock.testmock.MockTest)
\----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vagrant/cpython/Lib/unittest/test/testmock/testmock.py", line 2173, in test_property_not_called_with_spec_mock
    self.assertIsNone(obj._instance)
AssertionError: 'object' is not None

[1]

cpython/Lib/unittest/mock.py

Line 492 in fb27187

if iscoroutinefunction(getattr(spec, attr, None)):

[terryjreedy]

Without lines numbers, I cannot test which of the two identical asserts failed. Either comments or msg arguments will differentiate. Nor can I run snippets from two different files. Here is a minimal reproducible self-contained code that demonstrates the claim (which I verified on 3.9 and current master).

import unittest
from unittest.mock import Mock

class SomethingElse(object):
    def __init__(self):
        self._instance = None

    @property
    def instance(self):
        if not self._instance:
            self._instance = 'object'

class Test(unittest.TestCase):

    def test_property_not_called_with_spec_mock(self):
        obj = SomethingElse()
        self.assertIsNone(obj._instance, msg='before') # before
        mock = Mock(spec=obj)
        self.assertIsNone(obj._instance, msg='after') # after

unittest.main()

[sobolevn]

Related PR: #29901