unittest.mock.patch: Double patching instance method: AttributeError: Mock object has no attribute '__name__'

[asottile]

BPO	26704
Nosy	@cjw296, @voidspace, @asottile, @tirkarthi
PRs	bpo-26704: Add test for double patching instance methods #11085 [3.7] bpo-26704 - Add test for double patching instance methods (GH-11085) #11126
Files	patch patch2 2.patch

^{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 = <Date 2018-12-12.09:01:53.837>
created_at = <Date 2016-04-06.20:01:26.286>
labels = ['type-bug', 'library']
title = "unittest.mock.patch: Double patching instance method: AttributeError: Mock object has no attribute '__name__'"
updated_at = <Date 2018-12-12.09:01:53.837>
user = 'https://bugs.python.org/asottile'

bugs.python.org fields:

activity = <Date 2018-12-12.09:01:53.837>
actor = 'cjw296'
assignee = 'none'
closed = True
closed_date = <Date 2018-12-12.09:01:53.837>
closer = 'cjw296'
components = ['Library (Lib)']
creation = <Date 2016-04-06.20:01:26.286>
creator = 'asottile'
dependencies = []
files = ['42384', '42387', '42675']
hgrepos = []
issue_num = 26704
keywords = ['patch']
message_count = 9.0
messages = ['262960', '262966', '262972', '264611', '331430', '331536', '331579', '331588', '331590']
nosy_count = 5.0
nosy_names = ['cjw296', 'michael.foord', 'asottile', 'Anthony Sottile', 'xtreak']
pr_nums = ['11085', '11126']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26704'
versions = ['Python 3.5', 'Python 3.6']

[asottile]

Originally from testing-cabal/mock#350

## Example

from unittest import mock

class C(object):
    def f(self):
        pass

c = C()

with mock.patch.object(c, 'f', autospec=True):
    with mock.patch.object(c, 'f', autospec=True):
        pass

## Python3.3

$ test.py
$

## Python3.4 / 3.5 / 3.6 (From gitbhub.com/python/cpython@fa3fc6d7)

Traceback (most recent call last):
  File "test.py", line 10, in <module>
    with mock.patch.object(c, 'f', autospec=True):
  File "/home/asottile/workspace/cpython/Lib/unittest/mock.py", line 1320, in __enter__
    _name=self.attribute, **kwargs)
  File "/home/asottile/workspace/cpython/Lib/unittest/mock.py", line 2220, in create_autospec
    _check_signature(original, new, skipfirst=skipfirst)
  File "/home/asottile/workspace/cpython/Lib/unittest/mock.py", line 112, in _check_signature
    _copy_func_details(func, checksig)
  File "/home/asottile/workspace/cpython/Lib/unittest/mock.py", line 117, in _copy_func_details
    funcopy.__name__ = func.__name__
  File "/home/asottile/workspace/cpython/Lib/unittest/mock.py", line 578, in __getattr__
    raise AttributeError("Mock object has no attribute %r" % name)
AttributeError: Mock object has no attribute '__name__'

[asottile]

The root cause seems to be that autospecced functions return a function object (not a Mock instance) which a '.mock' attribute which is a MagicMock ( assigned here:

cpython/Lib/unittest/mock.py

Line 198 in ae775ab

funcopy.mock = mock

)

I took a first stab at a patch (attached)

[asottile]

Here's an improved patch which:

• passes the tests
• puts the test in the correct place

I'm not entirely happy with the approach -- open to suggestions :)

[asottile]

Seems I've named the patchfile incorrectly -- Hopefully this is correct this time?

[tirkarthi]

Thanks @asottile for the patch. I think the original AttributeError is resolved with bpo-28919 where they were silenced. It seems similar to bpo-32153 though the exception occurs from mock instead of partial object as in bpo-32153. The fix was applied to 3.7+ and hence 3.6 was not fixed which enters security fix only mode shortly. The attached tests also pass on master and I think it will it be a good unittest addition to Lib/unittest/test/testmock/testwith.py (similar to bpo-32153) that has a couple of nested with statements for the same attribute. Adding cjw296 to the list.

[asottile]

I've opened a PR with the test included: #11085

[cjw296]

Before we get too far: what's the use case for this double patching?

[asottile]

to be honest, I don't recall exactly given it's been 2 and a half years since the original report with no activity.

if I recall correctly, this was encountered while upgrading the mock backport in yelp's monolithic repository.

I want to say the reason this was hard to "fix" properly was due to some blanket patches being applied in a base test case and then other test cases re-patching those methods to add more specific behaviour. This worked fine in python2.7 and all the way until python3.3 but then was broken by changes in python3.4

Fortunately, they've been fixed in python3.7. I guess I've been encouraged to write a patch with a test so it does not regress in the future

[cjw296]

Ah, yeah, I can see the blanket patch and a more local patch in a monorepo being a thing, cool, let's have a look!