pytester: use monkeypatch fixture

Fixes #6213.

Author: blueyed

changelog/6213.improvement.rst

@@ -0,0 +1 @@
+pytester: the ``testdir`` fixture uses the ``monkeypatch`` fixture for changing the environment.

src/_pytest/pytester.py

@@ -538,7 +538,7 @@ def __init__(self, request, tmpdir_factory):
         self.request.addfinalizer(self.finalize)
         self._method = self.request.config.getoption("--runpytest")
 
-        mp = self.monkeypatch = MonkeyPatch()
+        mp = self.monkeypatch = self.request.getfixturevalue("monkeypatch")
         mp.setenv("PYTEST_DEBUG_TEMPROOT", str(self.test_tmproot))
         # Ensure no unexpected caching via tox.
         mp.delenv("TOX_ENV_DIR", raising=False)
@@ -566,7 +566,6 @@ def finalize(self):
         self._sys_modules_snapshot.restore()
         self._sys_path_snapshot.restore()
         self._cwd_snapshot.restore()
-        self.monkeypatch.undo()
 
     def __take_sys_modules_snapshot(self):
         # some zope modules used by twisted-related tests keep internal state

testing/test_pytester.py

@@ -542,17 +542,17 @@ def test_no_matching(function):
     func(bad_pattern)  # bad pattern does not match any line: passes
 
 
-def test_pytester_addopts(request, monkeypatch):
+def test_pytester_addopts_before_testdir(request, monkeypatch):
+    orig = os.environ.get("PYTEST_ADDOPTS", None)
     monkeypatch.setenv("PYTEST_ADDOPTS", "--orig-unused")
-
     testdir = request.getfixturevalue("testdir")
+    assert "PYTEST_ADDOPTS" not in os.environ
+    testdir.finalize()
+    assert os.environ.get("PYTEST_ADDOPTS") == orig
 
-    try:
-        assert "PYTEST_ADDOPTS" not in os.environ
-    finally:
-        testdir.finalize()
 
-    assert os.environ["PYTEST_ADDOPTS"] == "--orig-unused"
+def test_testdir_respects_monkeypatch(testdir, monkeypatch):
+    assert monkeypatch is testdir.monkeypatch
 
 
 def test_run_stdin(testdir):