Title: Bug in mock running on PyPy3
Type: Stage: resolved
Components: Versions:
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: cjw296 Nosy List: Carl.Friedrich.Bolz, cjw296
Priority: normal Keywords: patch

Created on 2020-01-29 13:49 by Carl.Friedrich.Bolz, last changed 2020-01-29 16:20 by cjw296. This issue is now closed.

Pull Requests
URL Status Linked Edit
PR 18252 merged Carl.Friedrich.Bolz, 2020-01-29 14:09
PR 18255 merged miss-islington, 2020-01-29 15:44
PR 18256 merged miss-islington, 2020-01-29 15:44
Messages (5)
msg360961 - (view) Author: Carl Friedrich Bolz-Tereick (Carl.Friedrich.Bolz) * Date: 2020-01-29 13:49
One of the new-in-3.8 tests for unittest.mock, test_spec_has_descriptor_returning_function, is failing on PyPy. This exposes a bug in unittest.mock. The bug is most noticeable on PyPy, where it can be triggered by simply writing a slightly weird descriptor (CrazyDescriptor in the test). Getting it to trigger on CPython would be possible too, by implementing the same descriptor in C, but I did not actually do that.

The relevant part of the test looks like this:

from unittest.mock import create_autospec

class CrazyDescriptor(object):
    def __get__(self, obj, type_):
        if obj is None:
            return lambda x: None

class MyClass(object):

    some_attr = CrazyDescriptor()

mock = create_autospec(MyClass)

On CPython this just works, on PyPy it fails with:

Traceback (most recent call last):
  File "", line 13, in <module>
  File "/home/cfbolz/bin/.pyenv/versions/pypy3.6-7.2.0/lib-python/3/unittest/", line 938, in __call__
    _mock_self._mock_check_sig(*args, **kwargs)
  File "/home/cfbolz/bin/.pyenv/versions/pypy3.6-7.2.0/lib-python/3/unittest/", line 101, in checksig
    sig.bind(*args, **kwargs)
  File "/home/cfbolz/bin/.pyenv/versions/pypy3.6-7.2.0/lib-python/3/", line 3034, in bind
    return args[0]._bind(args[1:], kwargs)
  File "/home/cfbolz/bin/.pyenv/versions/pypy3.6-7.2.0/lib-python/3/", line 2955, in _bind
    raise TypeError('too many positional arguments') from None
TypeError: too many positional arguments

The reason for this problem is that mock deduced that MyClass.some_attr is a method on PyPy. Since mock thinks the lambda returned by the descriptor is a method, it adds self as an argument, which leads to the TypeError.

Checking whether something is a method is done by _must_skip in The relevant condition is this one:

        elif isinstance(getattr(result, '__get__', None), MethodWrapperTypes):
            # Normal method => skip if looked up on type
            # (if looked up on instance, self is already skipped)
            return is_type
            return False

MethodWrapperTypes is defined as:

MethodWrapperTypes = (

which is just types.MethodType on PyPy, because there is no such thing as a method wrapper (the builtin types look pretty much like python-defined types in PyPy).

On PyPy the condition isinstance(getattr...) is thus True for all descriptors! so as soon as result has a __get__, it counts as a method, even in the above case where it's a custom descriptor.

Now even on CPython the condition makes no sense to me. It would be True for a C-defined version of CrazyDescriptor, it's just not a good way to check whether result is a method.

I would propose to replace the condition with the much more straightforward check:

        elif isinstance(result, FunctionTypes):

something is a method if it's a function on the class. Doing that change makes the test pass on PyPy, and doesn't introduce any test failures on CPython either.

Will open a pull request.
msg360968 - (view) Author: Chris Withers (cjw296) * (Python committer) Date: 2020-01-29 15:43
New changeset a327677905956ae0b239ff430a1346dfe265709e by Carl Friedrich Bolz-Tereick in branch 'master':
bpo-39485: fix corner-case in method-detection of mock (GH-18252)
msg360969 - (view) Author: Chris Withers (cjw296) * (Python committer) Date: 2020-01-29 16:10
New changeset cf0645a17acbc0c4dbbf82434e37637965748bbb by Miss Islington (bot) in branch '3.7':
bpo-39485: fix corner-case in method-detection of mock (GH-18256)
msg360970 - (view) Author: Chris Withers (cjw296) * (Python committer) Date: 2020-01-29 16:15
New changeset 696d2324cf2a54e20e8d6a6739fa97ba815a8be9 by Miss Islington (bot) in branch '3.8':
bpo-39485: fix corner-case in method-detection of mock (GH-18255)
msg360971 - (view) Author: Chris Withers (cjw296) * (Python committer) Date: 2020-01-29 16:20
Thank you very much for this, that was a really good catch!
Date User Action Args
2020-01-29 16:20:19cjw296setstatus: open -> closed
resolution: fixed
messages: + msg360971

stage: patch review -> resolved
2020-01-29 16:15:39cjw296setmessages: + msg360970
2020-01-29 16:10:31cjw296setmessages: + msg360969
2020-01-29 15:44:36miss-islingtonsetpull_requests: + pull_request17633
2020-01-29 15:44:19miss-islingtonsetpull_requests: + pull_request17632
2020-01-29 15:43:54cjw296setmessages: + msg360968
2020-01-29 15:37:56cjw296setassignee: cjw296
2020-01-29 14:09:08Carl.Friedrich.Bolzsetkeywords: + patch
stage: patch review
pull_requests: + pull_request17629
2020-01-29 13:49:55Carl.Friedrich.Bolzcreate