Raise an error if mocker is used in a with context

Ref: #164

src/pytest_mock/plugin.py

@@ -156,8 +156,22 @@ def _start_patch(self, mock_func, *args, **kwargs):
 
         def object(self, *args, **kwargs):
             """API to mock.patch.object"""
+            self._enforce_no_with_context(inspect.stack())
             return self._start_patch(self.mock_module.patch.object, *args, **kwargs)
 
+        def _enforce_no_with_context(self, stack):
+            """raises a ValueError if mocker is used in a with context"""
+            caller = stack[1]
+            frame = caller[0]
+            info = inspect.getframeinfo(frame)
+            code_context = " ".join(info.code_context).strip()
+
+            if "with mocker" in code_context:
+                raise ValueError(
+                    "Using mocker in a with context is not supported. "
+                    "https://github.com/pytest-dev/pytest-mock#note-about-usage-as-context-manager"
+                )
+
         def multiple(self, *args, **kwargs):
             """API to mock.patch.multiple"""
             return self._start_patch(self.mock_module.patch.multiple, *args, **kwargs)

tests/test_pytest_mock.py

@@ -721,3 +721,22 @@ def test_get_random_number(mocker):
     result = testdir.runpytest_subprocess()
     result.stdout.fnmatch_lines("* 1 passed in *")
     assert "RuntimeError" not in result.stderr.str()
+
+
+def test_abort_context_manager(mocker):
+    class A(object):
+        def doIt(self):
+            return False
+
+    a = A()
+
+    with pytest.raises(ValueError) as excinfo:
+        with mocker.patch.object(a, "doIt", return_value=True):
+            assert a.doIt() == True
+
+    expected_error_msg = (
+        "Using mocker in a with context is not supported. "
+        "https://github.com/pytest-dev/pytest-mock#note-about-usage-as-context-manager"
+    )
+
+    assert str(excinfo.value) == expected_error_msg