[PR #14196/8950ca3b backport][9.0.x] subtests: fix inconcistent handling of non-string messages

AUTHORS

@@ -382,6 +382,7 @@ Ralf Schmitt
 Ralph Giles
 Ram Rachum
 Ran Benita
+Randy Döring
 Raphael Castaneda
 Raphael Pierzina
 Rafal Semik

changelog/14195.bugfix.rst

@@ -0,0 +1 @@
+Fixed an issue where non-string messages passed to `unittest.TestCase.subTest()` were not printed.

src/_pytest/unittest.py

@@ -409,6 +409,10 @@ def addSubTest(
         | tuple[type[BaseException], BaseException, TracebackType]
         | None,
     ) -> None:
+        # Importing this private symbol locally in case this symbol is renamed/removed in the future; importing
+        # it globally would break pytest entirely, importing it locally only will break unittests using `addSubTest`.
+        from unittest.case import _subtest_msg_sentinel  # type: ignore[attr-defined]
+
         exception_info: ExceptionInfo[BaseException] | None
         match exc_info:
             case tuple():
@@ -427,7 +431,7 @@ def addSubTest(
             when="call",
             _ispytest=True,
         )
-        msg = test._message if isinstance(test._message, str) else None  # type: ignore[attr-defined]
+        msg = None if test._message is _subtest_msg_sentinel else str(test._message)  # type: ignore[attr-defined]
         report = self.ihook.pytest_runtest_makereport(item=self, call=call_info)
         sub_report = SubtestReport._new(
             report,

testing/test_subtests.py

@@ -370,6 +370,36 @@ def test_foo(subtests):
     )
 
 
+def test_msg_not_a_string(
+    pytester: pytest.Pytester, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """
+    Using a non-string in subtests.test() should still show it in the terminal (#14195).
+
+    Note: this was not a problem originally with the subtests fixture, only with TestCase.subTest; this test
+    was added for symmetry.
+    """
+    monkeypatch.setenv("COLUMNS", "120")
+    pytester.makepyfile(
+        """
+        def test_int_msg(subtests):
+            with subtests.test(42):
+                assert False, "subtest failure"
+
+        def test_no_msg(subtests):
+            with subtests.test():
+                assert False, "subtest failure"
+        """
+    )
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "SUBFAILED[[]42[]] test_msg_not_a_string.py::test_int_msg - AssertionError: subtest failure",
+            "SUBFAILED(<subtest>) test_msg_not_a_string.py::test_no_msg - AssertionError: subtest failure",
+        ]
+    )
+
+
 @pytest.mark.parametrize("flag", ["--last-failed", "--stepwise"])
 def test_subtests_last_failed_step_wise(pytester: pytest.Pytester, flag: str) -> None:
     """Check that --last-failed and --step-wise correctly rerun tests with failed subtests."""
@@ -619,6 +649,33 @@ def test_foo(self):
             "SUBSKIPPED[[]subtest 1[]] [[]1[]] *.py:*: skip subtest 1"
         )
 
+    def test_msg_not_a_string(
+        self, pytester: pytest.Pytester, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Using a non-string in TestCase.subTest should still show it in the terminal (#14195)."""
+        monkeypatch.setenv("COLUMNS", "120")
+        pytester.makepyfile(
+            """
+            from unittest import TestCase
+
+            class T(TestCase):
+                def test_int_msg(self):
+                    with self.subTest(42):
+                        assert False, "subtest failure"
+
+                def test_no_msg(self):
+                    with self.subTest():
+                        assert False, "subtest failure"
+            """
+        )
+        result = pytester.runpytest()
+        result.stdout.fnmatch_lines(
+            [
+                "SUBFAILED[[]42[]] test_msg_not_a_string.py::T::test_int_msg - AssertionError: subtest failure",
+                "SUBFAILED(<subtest>) test_msg_not_a_string.py::T::test_no_msg - AssertionError: subtest failure",
+            ]
+        )
+
 
 class TestCapture:
     def create_file(self, pytester: pytest.Pytester) -> None: