allow passing mock name to mocker.stub()

This allows customizing the stub's name displayed in any assertion failure
messages (useful for debugging), without having to resort to setting `Mock`
object's internal `_mock_name` attribute directly.

README.rst

@@ -119,14 +119,15 @@ Stub
 *New in version 0.6*
 
 The stub is a mock object that accepts any arguments and is useful to test callbacks, for instance.
+May be passed a name to be used by the constructed stub object in its repr (useful for debugging).
 
 .. code-block:: python
 
     def test_stub(mocker):
         def foo(on_something):
             on_something('foo', 'bar')
 
-        stub = mocker.stub()
+        stub = mocker.stub(name='on_something_stub')
 
         foo(stub)
         stub.assert_called_once_with('foo', 'bar')

pytest_mock.py

@@ -70,15 +70,16 @@ def spy(self, obj, name):
                                    autospec=autospec)
         return result
 
-    def stub(self):
+    def stub(self, name=None):
         """
         Creates a stub method. It accepts any arguments. Ideal to register to
         callbacks in tests.
 
+        :param name: the constructed stub's name as used in repr
         :rtype: mock.MagicMock
         :return: Stub object.
         """
-        return mock_module.MagicMock(spec=lambda *args, **kwargs: None)
+        return mock_module.MagicMock(spec=lambda *args, **kwargs: None, name=name)
 
     class _Patcher(object):
         """

test_pytest_mock.py

@@ -168,14 +168,35 @@ def test_mocker_resetall(mocker):
     assert not open.called
 
 
-def test_mocker_stub(mocker):
-    def foo(on_something):
-        on_something('foo', 'bar')
-
-    stub = mocker.stub()
-
-    foo(stub)
-    stub.assert_called_once_with('foo', 'bar')
+class TestMockerStub:
+    def test_call(self, mocker):
+        stub = mocker.stub()
+        stub('foo', 'bar')
+        stub.assert_called_once_with('foo', 'bar')
+
+    def test_repr_with_no_name(self, mocker):
+        stub = mocker.stub()
+        assert not 'name' in repr(stub)
+
+    def test_repr_with_name(self, mocker):
+        test_name = 'funny walk'
+        stub = mocker.stub(name=test_name)
+        assert "name={!r}".format(test_name) in repr(stub)
+
+    def __test_failure_message(self, mocker, **kwargs):
+        expected_name = kwargs.get('name') or 'mock'
+        expected_message = 'Expected call: {}()\nNot called'.format(expected_name)
+        stub = mocker.stub(**kwargs)
+        with pytest.raises(AssertionError) as exc_info:
+            stub.assert_called_with()
+        assert exc_info.value.msg == expected_message
+
+    def test_failure_message_with_no_name(self, mocker):
+        self.__test_failure_message(mocker)
+
+    @pytest.mark.parametrize('name', (None, '', 'f', 'The Castle of aaarrrrggh'))
+    def test_failure_message_with_name(self, mocker, name):
+        self.__test_failure_message(mocker, name=name)
 
 
 def test_instance_method_spy(mocker):