WIP: initial repro for #8412

testing/example_scripts/order_issue.py

@@ -0,0 +1,45 @@
+import os
+import unittest
+
+import pytest
+
+
+class EnvironmentAwareMixin:
+    @pytest.fixture(autouse=True)
+    def _monkeypatch(self, monkeypatch):
+        self._envpatcher = monkeypatch
+
+    def set_environ(self, name, value):
+        self._envpatcher.setenv(name, value)
+
+
+# This arrangement fails: setup_method runs before _monkeypatch
+class MyPytestBase(
+    EnvironmentAwareMixin,
+):
+    pass
+
+
+class TestSomething(MyPytestBase):
+    @pytest.fixture(autouse=True)
+    def setup_method(self):
+        self.set_environ("X", "1")
+
+    def test_something(self):
+        assert os.environ["X"] == "1"
+
+
+# This arrangement works: _monkeypatch runs before setUp
+class MyUnittestBase(
+    EnvironmentAwareMixin,
+    unittest.TestCase,
+):
+    pass
+
+
+class TestSomethingElse(MyUnittestBase):
+    def setUp(self):
+        self.set_environ("X", "1")
+
+    def test_something_else(self):
+        assert os.environ["X"] == "1"

testing/examples/test_order_issue.py

@@ -0,0 +1,7 @@
+from _pytest.pytester import Pytester
+
+
+def test_order(pytester: Pytester) -> None:
+    pytester.copy_example("order_issue.py")
+    rep = pytester.runpytest("order_issue.py")
+    rep.assert_outcomes(passed=2)