无法在python中模拟__subclasses__

Question

Why does the following code not work?

>>> from mock import *
>>> class A(object):
...     pass
...
>>> mock = create_autospec(A)
>>> mock.foo = Mock()                # this works
>>> mock.__bar__ = Mock()            # this works too
>>> mock.__subclasses__ = Mock()     # this fails
AttributeError: Mock object has no attribute '__subclasses__'

I think I'm following the documentation on mocking magic methods here. The docs do remark that trying to mock a magic method which is not in the spec will not work. But why would __subclasses__ not be in the spec of an autospecced new-style class?

Answer 1

__subclasses__ is not part of the class spec. It is part of the metatype of the class ( type here).

Python always looks up special methods on the type , never directly. If and when Python needs to call __subclasses__ , it'll not do so directly, it'll use type(classobj).__subclasses__(classobj) to look up the unbound method and pass in the first argument manually. As such, adding __subclasses__ to the mock of a class is not going to be enough.

The same applies to special methods intended to operate on instances; Mock will happily take __add__ or __str__ along when using create_autospec() , and that works then for instances of the mocked class where Python will use type(mockinstance).__str__(mockinstance) to invoke the __str__ method.

If your own code calls classobj.__subclasses__() directly, you'll need to pass in a class mock that explicitly sets that method; you can't expect it to be auto-specced here.

Mocks don't like you setting any valid magic method:

>>> m = create_autospec(A)
>>> m.__add__ = Mock()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/venvs/stackoverflow-2.7/lib/python2.7/site-packages/mock.py", line 767, in __setattr__
    raise AttributeError("Mock object has no attribute '%s'" % name)
AttributeError: Mock object has no attribute '__add__'

only non-default magic methods are allowed:

>>> m.__dummy__ = Mock()
>>> m.__dummy__
<Mock name='mock.__dummy__' id='4427608656'>

You can create a subclass of A adding in the __subclass__ method to allow you to mock it:

>>> class AMockSpec(A):
...     def __subclasses__(self): pass
...
>>> m = create_autospec(AMockSpec)
>>> m.__subclasses__ = Mock()
>>> m.__subclasses__.return_value = ['SomeMockValue']
>>> m.__subclasses__()
['SomeMockValue']

Answer 2

The following does work ( __subclasses__ is available on __class__ ):

>>> from mock import *
>>> class A(object):
...     pass
...
>>> m = create_autospec(A)
>>> m.__class__.__subclasses__()
[]
>>> class B(A): pass
...
>>> m.__class__.__subclasses__()
[<class '__main__.B'>]
>>> m.__class__.__subclasses__ = Mock()
>>> m.__class__.__subclasses__()
<Mock name='mock()' id='4372594896'>