mock raises exception when using a spec with an attribute that raises exception on access

[kjamieson]

BPO	45756
Nosy	@ambv, @hongweipeng, @tirkarthi, @sobolevn, @jaap3
PRs	bpo-45756: do not execute @property descrs while creating mock autospecs #29901 bpo-45756: Fix mock triggers dynamic lookup via the descriptor protocol. #31348

^{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 2021-11-08.19:47:53.494>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.11']
title = 'mock raises exception when using a spec with an attribute that raises exception on access'
updated_at = <Date 2022-02-15.06:36:26.841>
user = 'https://bugs.python.org/kjamieson'

bugs.python.org fields:

activity = <Date 2022-02-15.06:36:26.841>
actor = 'hongweipeng'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-11-08.19:47:53.494>
creator = 'kjamieson'
dependencies = []
files = []
hgrepos = []
issue_num = 45756
keywords = ['patch']
message_count = 3.0
messages = ['405982', '406284', '406452']
nosy_count = 6.0
nosy_names = ['lukasz.langa', 'hongweipeng', 'xtreak', 'sobolevn', 'kjamieson', 'jaap3']
pr_nums = ['29901', '31348']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45756'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

[kjamieson]

In Python 3.8 and later creating a mock with a spec specifying an object containing a property that happens to raise an exception when accessed will fail, because _mock_add_spec calls getattr() on every attribute of the spec. This did not happen in Python 3.6/3.7.

This is likely a fairly unusual scenario (and in the particular case where I encountered this I could just use a class instead of an instance for the spec), but it was surprising.

For example:

# cat test.py
from unittest import mock

class Foo:
    @property
    def bar(self) -> str:
        raise Exception('xxx')

m = mock.MagicMock(spec=Foo())

# python3.11 test.py
Traceback (most recent call last):
  File "/root/test.py", line 8, in <module>
    m = mock.MagicMock(spec=Foo())
        ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/mock.py", line 2069, in __init__
    _safe_super(MagicMixin, self).__init__(*args, **kw)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/mock.py", line 1087, in __init__
    _safe_super(CallableMixin, self).__init__(
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/mock.py", line 442, in __init__
    self._mock_add_spec(spec, spec_set, _spec_as_instance, _eat_self)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/mock.py", line 497, in _mock_add_spec
    if iscoroutinefunction(getattr(spec, attr, None)):
                           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/test.py", line 6, in bar
    raise Exception('xxx')
    ^^^^^^^^^^^^^^^^^^^^^^
Exception: xxx

[jaap3]

I think I encountered this when trying to mock requests.Response: psf/requests#5944

[tirkarthi]

This seems to be similar to https://bugs.python.org/issue41768 .