How to let MagicMock behave like a dict?

Why mess around with __iter__? It seems to me that you want to mess with __contains__:

>>> import mock
>>> m = mock.MagicMock()
>>> d = {'foo': 'bar'}
>>> m.__getitem__.side_effect = d.__getitem__
>>> m.__iter__.side_effect = d.__iter__
>>> m['foo']
'bar'
>>> 'foo' in m
False
>>> m.__contains__.side_effect = d.__contains__
>>> 'foo' in m
True