#3290 Fix comments

_pytest/monkeypatch.py

@@ -110,6 +110,21 @@ class MonkeyPatch(object):
 
     @contextmanager
     def context(self):
+        """
+        Context manager that returns a new :class:`MonkeyPatch` object which
+        undoes any patching done inside the ``with`` block upon exit:
+
+        .. code-block:: python
+
+            import functools
+            def test_partial(monkeypatch):
+                with monkeypatch.context() as m:
+                    m.setattr(functools, "partial", 3)
+
+        Useful in situations where it is desired to undo some patches before the test ends,
+        such as mocking ``stdlib`` functions that might break pytest itself if mocked (for examples
+        of this see `#3290 <https://github.com/pytest-dev/pytest/issues/3290>`_.
+        """
         m = MonkeyPatch()
         try:
             yield m

changelog/3290.bugfix

@@ -1,2 +0,0 @@
-Improved `monkeypatch` to support some form of with statement. Now you can use `with monkeypatch.context() as m:`
-construction to avoid damage of Pytest.

changelog/3290.feature

@@ -0,0 +1,2 @@
+``monkeypatch`` now supports a ``context()`` function which acts as a context manager which undoes all patching done
+within the ``with`` block.

doc/en/monkeypatch.rst

@@ -62,16 +62,20 @@ so that any attempts within tests to create http requests will fail.
     unavoidable, passing ``--tb=native``, ``--assert=plain`` and ``--capture=no`` might 
     help although there's no guarantee.
 
-    To avoid damage of pytest from patching python stdlib functions use ``with``
-    construction::
+.. note::
 
-        # content of test_module.py
+    Mind that patching ``stdlib`` functions and some third-party libraries used by pytest
+    might break pytest itself, therefore in those cases it is recommended to use
+    :meth:`MonkeyPatch.context` to limit the patching to the block you want tested:
 
+    .. code-block:: python
         import functools
         def test_partial(monkeypatch):
             with monkeypatch.context() as m:
                 m.setattr(functools, "partial", 3)
                 assert functools.partial == 3
+
+    See issue `#3290 <https://github.com/pytest-dev/pytest/issues/3290>`_ for details.
     
 
 .. currentmodule:: _pytest.monkeypatch

testing/test_monkeypatch.py

@@ -329,14 +329,13 @@ def test_issue1338_name_resolving():
         monkeypatch.undo()
 
 
-def test_context(testdir):
-    testdir.makepyfile("""
-    import functools
-    def test_partial(monkeypatch):
-        with monkeypatch.context() as m:
-            m.setattr(functools, "partial", 3)
-            assert functools.partial == 3
-    """)
+def test_context():
+    monkeypatch = MonkeyPatch()
 
-    result = testdir.runpytest()
-    result.stdout.fnmatch_lines("*1 passed*")
+    import functools
+    import inspect
+
+    with monkeypatch.context() as m:
+        m.setattr(functools, "partial", 3)
+        assert not inspect.isclass(functools.partial)
+    assert inspect.isclass(functools.partial)