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I guess this should be expected... too much magic :P
>>> from unittest.mock import MagicMock
>>> MagicMock(**{'__hash__.return_value': "FIXME"})
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/nix/store/qlvbf3n3y34idxcgwwhsi9pq26v28q99-python3-3.4.2/lib/python3.4/unittest/mock.py", line 1772, in __init__
 _safe_super(MagicMixin, self).__init__(*args, **kw)
 File "/nix/store/qlvbf3n3y34idxcgwwhsi9pq26v28q99-python3-3.4.2/lib/python3.4/unittest/mock.py", line 881, in __init__
 _spec_state, _new_name, _new_parent, **kwargs
 File "/nix/store/qlvbf3n3y34idxcgwwhsi9pq26v28q99-python3-3.4.2/lib/python3.4/unittest/mock.py", line 410, in __init__
 self.configure_mock(**kwargs)
 File "/nix/store/qlvbf3n3y34idxcgwwhsi9pq26v28q99-python3-3.4.2/lib/python3.4/unittest/mock.py", line 560, in configure_mock
 setattr(obj, final, val)
AttributeError: 'method-wrapper' object has no attribute 'return_value'
The same happens with e.g. __str__
>>> m.configure_mock(**{'__hash__.return_value': 1})
works just fine, instead

mock itself fails

Tests for this bug + proposed fix.

For Mock both the following tests fail, I would say it is an expected behavior.
def test_setting_magic_method_for_mock(self):
 m = Mock()
 m.configure_mock(**{'__str__.return_value': "14"})
 self.assertEqual(str(m), "14")
 
def test_setting_magic_method_in_mock_initialization(self):
 m = Mock(**{'__str__.return_value': "12"})
 self.assertEqual(str(m), "12")

Added more logging to the patch.

New changeset 4d1236b180be by Łukasz Langa in branch 'default':
Issue #23310: Fix MagicMock's initializer to work with __methods__.
https://hg.python.org/cpython/rev/4d1236b180be 

New changeset 8e5592a43d65 by Łukasz Langa in branch '3.4':
Issue #23310: Fix MagicMock's initializer to work with __methods__.
https://hg.python.org/cpython/rev/8e5592a43d65
New changeset dd8f48ff9480 by Łukasz Langa in branch 'default':
Merge 3.4 (#23310)
https://hg.python.org/cpython/rev/dd8f48ff9480 

Awesome! Thank you for your patch, Kasia. For the record, I left the non-magic Mock behavior untouched since Kasia rightfully points out that in this case m.configure_mock() also does not work.