How to patch classmethod with autospec in unmocked class?

Question

I want to assert that one classmethod in a Python class calls another classmethod with a certain set of arguments. I would like the mocked classmethod to be "spec-ed", so it detects if it is called with the wrong number of arguments.

When I patch the classmethod using patch.object(.., autospec=True, ..), the classmethod is replaced with a NonCallableMagicMock and raises an error when I try to call it.

from mock import patch

class A(object):

    @classmethod
    def api_meth(cls):
        return cls._internal_classmethod(1, 2, 3)

    @classmethod
    def _internal_classmethod(cls, n, m, o):
        return sum(n, m, o)

with patch.object(A, '_internal_classmethod') as p:
    print(type(p).__name__)

with patch.object(A, '_internal_classmethod', autospec=True) as p:
    print(type(p).__name__)

produces the output:

MagicMock
NonCallableMagicMock

How can I get a spec-ed mock for _internal_classmethod when the class it belongs to is not mocked?

Answer 1

There's an outstanding bug report (google code link and python bug tracker link) to fix this issue. Until the fix gets incorporated, you can try the following, which worked for me [On 2.7, though I think it would also work in 3.x].

def _patched_callable(obj):
    "Monkeypatch to allow autospec'ed classmethods and staticmethods."
    # See https://code.google.com/p/mock/issues/detail?id=241 and
    # http://bugs.python.org/issue23078 for the relevant bugs this
    # monkeypatch fixes
    if isinstance(obj, type):
        return True
    if getattr(obj, '__call__', None) is not None:
        return True
    if (isinstance(obj, (staticmethod, classmethod))
        and mock._callable(obj.__func__)):
        return True
    return False
_patched_callable._old_func = mock._callable
mock._callable = _patched_callable

After the monkeypatch, you should be able to use mock.patch normally and have static- and class-methods patched properly.

Answer 2

Use spec in place of autospec, and set it directly.

with patch.object(A, '_internal_classmethod', spec=A._internal_classmethod) as p:
    print(type(p).__name__)

gives me

MagicMock

for output.