Fix performance when raising ValueError when used as context-manager

CHANGELOG.rst

@@ -1,3 +1,11 @@
+3.1.1 (2020-05-31)
+------------------
+
+* Fixed performance regression caused by the ``ValueError`` raised
+  when ``mocker`` is used as context manager (`#191`_).
+
+.. _#191: https://github.com/pytest-dev/pytest-mock/issues/191
+
 3.1.0 (2020-04-18)
 ------------------
 

src/pytest_mock/plugin.py

@@ -158,30 +158,20 @@ def _start_patch(self, mock_func, *args, **kwargs):
             module, registering the patch to stop it later and returns the
             mock object resulting from the mock call.
             """
-            self._enforce_no_with_context(inspect.stack())
             p = mock_func(*args, **kwargs)
             mocked = p.start()
             self._patches.append(p)
             if hasattr(mocked, "reset_mock"):
                 self._mocks.append(mocked)
+                # check if `mocked` is actually a mock object, as depending on autospec or target
+                # parameters `mocked` can be anything
+                if hasattr(mocked, "__enter__"):
+                    mocked.__enter__.side_effect = ValueError(
+                        "Using mocker in a with context is not supported. "
+                        "https://github.com/pytest-dev/pytest-mock#note-about-usage-as-context-manager"
+                    )
             return mocked
 
-        def _enforce_no_with_context(self, stack):
-            """raises a ValueError if mocker is used in a with context"""
-            caller = stack[2]
-            frame = caller[0]
-            info = inspect.getframeinfo(frame)
-            if info.code_context is None:
-                # no source code available (#169)
-                return
-            code_context = " ".join(info.code_context).strip()
-
-            if code_context.startswith("with mocker."):
-                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 object(self, *args, **kwargs):
             """API to mock.patch.object"""
             return self._start_patch(self.mock_module.patch.object, *args, **kwargs)

tests/test_pytest_mock.py

@@ -773,7 +773,7 @@ def doIt(self):
 
 def test_abort_patch_context_manager(mocker):
     with pytest.raises(ValueError) as excinfo:
-        with mocker.patch("some_package"):
+        with mocker.patch("json.loads"):
             pass
 
     expected_error_msg = (
@@ -784,6 +784,26 @@ def test_abort_patch_context_manager(mocker):
     assert str(excinfo.value) == expected_error_msg
 
 
+def test_context_manager_patch_example(mocker):
+    """Our message about misusing mocker as a context manager should not affect mocking
+    context managers (see #192)"""
+
+    class dummy_module:
+        class MyContext:
+            def __enter__(self, *args, **kwargs):
+                return 10
+
+            def __exit__(self, *args, **kwargs):
+                pass
+
+    def my_func():
+        with dummy_module.MyContext() as v:
+            return v
+
+    m = mocker.patch.object(dummy_module, "MyContext")
+    assert isinstance(my_func(), mocker.MagicMock)
+
+
 def test_abort_patch_context_manager_with_stale_pyc(testdir):
     """Ensure we don't trigger an error in case the frame where mocker.patch is being
     used doesn't have a 'context' (#169)"""