pytester: testdir: setup monkeypatch via pytest_runtest_call (#192)

Ref: pytest-dev#6687

Ref: pytest-dev#6219
Ref: pytest-dev#5031

src/_pytest/pytester.py

@@ -15,6 +15,7 @@
 from io import StringIO
 from typing import Callable
 from typing import Dict
+from typing import Generator
 from typing import Iterable
 from typing import List
 from typing import Optional
@@ -39,6 +40,7 @@
 from _pytest.nodes import Collector
 from _pytest.nodes import Item
 from _pytest.pathlib import Path
+from _pytest.python import Function
 from _pytest.python import Module
 from _pytest.reports import TestReport
 from _pytest.tmpdir import TempdirFactory
@@ -535,6 +537,39 @@ def try_rel(arg: Path) -> Path:
     print("{}: {}\n{}in: {}".format(header, args_str, indent, cwd))
 
 
+@pytest.hookimpl(hookwrapper=True, trylast=True)
+def pytest_runtest_call(item: Function) -> Generator[None, None, None]:
+    """Setup/activate testdir's monkeypatching only during test calls.
+
+    When it would be done via the instance/fixture directly it would also be
+    active during teardown (e.g. with the terminal plugin's reporting), where
+    it might mess with the column width etc.
+    """
+    testdir = item.funcargs.get("testdir")
+    if not isinstance(testdir, Testdir):
+        yield
+        return
+
+    mp = testdir.monkeypatch
+    mp.setenv("PYTEST_DEBUG_TEMPROOT", str(testdir.test_tmproot))
+    # Ensure no unexpected caching via tox.
+    mp.delenv("TOX_ENV_DIR", raising=False)
+    # Discard outer pytest options.
+    mp.delenv("PYTEST_ADDOPTS", raising=False)
+    # Ensure no user config is used.
+    tmphome = str(testdir.tmpdir)
+    mp.setenv("HOME", tmphome)
+    mp.setenv("USERPROFILE", tmphome)
+    # Do not use colors for inner runs by default.
+    mp.setenv("PY_COLORS", "0")
+
+    mp.setattr("_pytest.terminal._cached_terminal_width", None)
+    try:
+        yield
+    finally:
+        mp.undo()
+
+
 class Testdir:
     """Temporary test directory with tools to test/run pytest itself.
 
@@ -575,20 +610,7 @@ def __init__(self, request: FixtureRequest, tmpdir_factory: TempdirFactory) -> N
         self.chdir()
         self.request.addfinalizer(self.finalize)
         self._method = self.request.config.getoption("--runpytest")
-
-        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)
-        # Discard outer pytest options.
-        mp.delenv("PYTEST_ADDOPTS", raising=False)
-        # Ensure no user config is used.
-        tmphome = str(self.tmpdir)
-        mp.setenv("HOME", tmphome)
-        mp.setenv("USERPROFILE", tmphome)
-        # Do not use colors for inner runs by default.
-        mp.setenv("PY_COLORS", "0")
-        mp.setattr("_pytest.terminal._cached_terminal_width", 80)
+        self.monkeypatch = MonkeyPatch()
 
     def __repr__(self):
         return "<Testdir {!r}>".format(self.tmpdir)

testing/test_pytester.py

@@ -573,17 +573,36 @@ def test_no_matching_after_match() -> None:
 
 
 def test_pytester_addopts_before_testdir(request, monkeypatch) -> None:
-    orig = os.environ.get("PYTEST_ADDOPTS", None)
     monkeypatch.setenv("PYTEST_ADDOPTS", "--orig-unused")
+    # Requesting the fixture dynamically / instantiating Testdir does not
+    # change the env via its monkeypatch instance.
     testdir = request.getfixturevalue("testdir")
-    assert "PYTEST_ADDOPTS" not in os.environ
+    assert os.environ["PYTEST_ADDOPTS"] == "--orig-unused"
+
+    assert testdir.monkeypatch is not monkeypatch
+    testdir.monkeypatch.setenv("PYTEST_ADDOPTS", "-p pytester")
+
+    p1 = testdir.makepyfile(
+        """
+        import os
+
+        def test(testdir):
+            assert "PYTEST_ADDOPTS" not in os.environ
+
+        def test2():
+            assert os.environ.get("PYTEST_ADDOPTS") == "-p pytester"
+        """
+    )
+    result = testdir.runpytest(str(p1))
+    assert result.ret == 0
+
+    # Finalizer undos always, although a context is used via
+    # pytest_runtest_call.
     testdir.finalize()
     assert os.environ.get("PYTEST_ADDOPTS") == "--orig-unused"
-    monkeypatch.undo()
-    assert os.environ.get("PYTEST_ADDOPTS") == orig
 
 
-def test_testdir_terminal_width(request, monkeypatch):
+def test_testdir_terminal_width(monkeypatch):
     """testdir does not set COLUMNS, but _cached_terminal_width."""
     from _pytest.terminal import get_terminal_width
 
@@ -595,18 +614,35 @@ def test_testdir_terminal_width(request, monkeypatch):
     assert get_terminal_width() == 1234
     monkeypatch.delenv("COLUMNS")
 
-    testdir = request.getfixturevalue("testdir")
-    assert get_terminal_width() == 80
-    assert "COLUMNS" not in os.environ
-    testdir.finalize()
-    assert get_terminal_width() == 1234
-    assert "COLUMNS" not in os.environ
-
     monkeypatch.undo()
     assert os.environ.get("COLUMNS") == orig_env
     assert get_terminal_width() == orig_width
 
 
+def test_testdir_terminal_width_inner(testdir, monkeypatch):
+    monkeypatch.setattr("_pytest.terminal._cached_terminal_width", 1234)
+    monkeypatch.delenv("COLUMNS", raising=False)
+
+    p1 = testdir.makepyfile(
+        """
+        import os
+        from _pytest.terminal import get_terminal_width
+
+        def test1():
+            assert get_terminal_width() == 1234
+
+        def test2(testdir):
+            assert get_terminal_width() == 80
+            assert "COLUMNS" not in os.environ
+            testdir.finalize()
+            assert get_terminal_width() == 1234
+            assert "COLUMNS" not in os.environ
+        """
+    )
+    result = testdir.runpytest("-p", "pytester", str(p1))
+    assert result.ret == 0
+
+
 def test_run_stdin(testdir) -> None:
     with pytest.raises(testdir.TimeoutExpired):
         testdir.run(