pytester: use monkeypatch fixture

Fixes pytest-dev#6213.

changelog/6213.improvement.rst

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

src/_pytest/pytester.py

@@ -359,8 +359,8 @@ def LineMatcher_fixture(request: FixtureRequest) -> "Type[LineMatcher]":
 
 
 @pytest.fixture
-def testdir(request: FixtureRequest, tmpdir_factory) -> "Testdir":
-    return Testdir(request, tmpdir_factory)
+def testdir(request: FixtureRequest, tmpdir_factory, monkeypatch) -> "Testdir":
+    return Testdir(request, tmpdir_factory, monkeypatch=monkeypatch)
 
 
 @pytest.fixture
@@ -524,21 +524,25 @@ class Testdir:
     class TimeoutExpired(Exception):
         pass
 
-    def __init__(self, request, tmpdir_factory):
+    def __init__(
+        self, request, tmpdir_factory, *, monkeypatch: Optional[MonkeyPatch] = None
+    ) -> None:
         self.request = request
-        self._mod_collections = WeakKeyDictionary()
+        self._mod_collections = WeakKeyDictionary()  # type: ignore
         name = request.function.__name__
         self.tmpdir = tmpdir_factory.mktemp(name, numbered=True)
         self.test_tmproot = tmpdir_factory.mktemp("tmp-" + name, numbered=True)
-        self.plugins = []
+        self.plugins = []  # type: ignore
         self._cwd_snapshot = CwdSnapshot()
         self._sys_path_snapshot = SysPathsSnapshot()
         self._sys_modules_snapshot = self.__take_sys_modules_snapshot()
         self.chdir()
         self.request.addfinalizer(self.finalize)
         self._method = self.request.config.getoption("--runpytest")
 
-        mp = self.monkeypatch = MonkeyPatch()
+        if monkeypatch is None:
+            monkeypatch = MonkeyPatch()
+        mp = self.monkeypatch = monkeypatch
         mp.setenv("PYTEST_DEBUG_TEMPROOT", str(self.test_tmproot))
         # Ensure no unexpected caching via tox.
         mp.delenv("TOX_ENV_DIR", raising=False)

testing/test_pytester.py

@@ -542,17 +542,26 @@ 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")
-
-    try:
-        assert "PYTEST_ADDOPTS" not in os.environ
-    finally:
-        testdir.finalize()
-
-    assert os.environ["PYTEST_ADDOPTS"] == "--orig-unused"
+    assert "PYTEST_ADDOPTS" not in os.environ
+    testdir.finalize()
+    assert os.environ.get("PYTEST_ADDOPTS") == orig
+
+
+@pytest.mark.parametrize("method", ("setenv", "delenv"))
+def test_testdir_respects_monkeypatch(method, testdir, monkeypatch):
+    assert monkeypatch is testdir.monkeypatch
+    assert testdir._env_run_update["COLUMNS"] == "80"
+    assert testdir._get_env_run_update()["COLUMNS"] == "80"
+    if method == "setenv":
+        monkeypatch.setenv("COLUMNS", "12")
+    else:
+        assert method == "delenv"
+        monkeypatch.delenv("COLUMNS", raising=False)
+    assert "COLUMNS" not in testdir._get_env_run_update()
 
 
 def test_run_stdin(testdir):