pytester: respect monkeypatch fixture for env updates

Fixes #6213.

Author: blueyed

changelog/6213.improvement.rst

@@ -0,0 +1 @@
+pytester: the ``testdir`` fixture respects environment settings from the ``monkeypatch`` fixture for inner runs.

src/_pytest/pytester.py

@@ -855,7 +855,7 @@ def inline_run(self, *args, plugins=(), no_reraise_ctrlc=False):
         try:
             # Do not load user config (during runs only).
             mp_run = MonkeyPatch()
-            for k, v in self._env_run_update.items():
+            for k, v in self._get_env_run_update().items():
                 mp_run.setenv(k, v)
             finalizers.append(mp_run.undo)
 
@@ -1091,7 +1091,7 @@ def popen(
         env["PYTHONPATH"] = os.pathsep.join(
             filter(None, [os.getcwd(), env.get("PYTHONPATH", "")])
         )
-        env.update(self._env_run_update)
+        env.update(self._get_env_run_update())
         kw["env"] = env
 
         if stdin is Testdir.CLOSE_STDIN:
@@ -1263,13 +1263,19 @@ def spawn(self, cmd, expect_timeout=10.0):
 
         # Do not load user config.
         env = os.environ.copy()
-        env.update(self._env_run_update)
+        env.update(self._get_env_run_update())
 
         child = pexpect.spawn(cmd, logfile=logfile, env=env)
         self.request.addfinalizer(logfile.close)
         child.timeout = expect_timeout
         return child
 
+    def _get_env_run_update(self):
+        """Get updates for environment not overridden via monkeypatch fixture."""
+        outer_mp = self.request.getfixturevalue("monkeypatch")
+        outer = {item[1] for item in outer_mp._setitem if item[0] is os.environ}
+        return {k: v for k, v in self._env_run_update.items() if k not in outer}
+
 
 def getdecoded(out):
     try:

testing/test_pytester.py

@@ -542,17 +542,27 @@ 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"
+@pytest.mark.parametrize("method", ("setenv", "delenv"))
+def test_testdir_respects_monkeypatch(method, testdir, monkeypatch):
+    assert monkeypatch is not testdir.monkeypatch
+    tmphome = str(testdir.tmpdir)
+    assert testdir._env_run_update["HOME"] == tmphome
+    assert testdir._get_env_run_update()["HOME"] == tmphome
+    if method == "setenv":
+        monkeypatch.setenv("HOME", "anotherhome")
+    else:
+        assert method == "delenv"
+        monkeypatch.delenv("HOME", raising=False)
+    assert "HOME" not in testdir._get_env_run_update()
 
 
 def test_run_stdin(testdir):