Fix serialization of str subclasses in subtest msg parameter

changelog/15134.bugfix.rst

@@ -0,0 +1 @@
+Fixed ``DumpError`` from ``pytest-xdist`` when a ``str`` subclass (such as a :class:`~enum.StrEnum` member) is passed as the ``msg`` of :fixture:`subtests` -- the message is now converted to a plain ``str``.

src/_pytest/subtests.py

@@ -67,6 +67,12 @@ def __post_init__(self) -> None:
         object.__setattr__(
             self, "kwargs", {k: saferepr(v) for (k, v) in self.kwargs.items()}
         )
+        # Ensure msg is a plain str: str subclasses such as StrEnum members
+        # cannot be serialized by execnet (pytest-dev/pytest-xdist#1161).
+        # str.__str__ keeps the string data, while str() on a (str, Enum)
+        # mixin would return "Cls.MEMBER".
+        if isinstance(self.msg, str) and type(self.msg) is not str:
+            object.__setattr__(self, "msg", str.__str__(self.msg))
 
     def _to_json(self) -> dict[str, Any]:
         result = dataclasses.asdict(self)

testing/test_subtests.py

@@ -1048,6 +1048,25 @@ def test_serialization() -> None:
     )
 
 
+class MyStrEnum(str, Enum):
+    B = "B"
+
+
+@pytest.mark.parametrize(
+    ("msg", "expected"),
+    [
+        pytest.param(MyStrEnum.B, "B", id="str-subclass"),
+        pytest.param("plain", "plain", id="str"),
+        pytest.param(None, None, id="none"),
+    ],
+)
+def test_msg_is_plain_str(msg: str | None, expected: str | None) -> None:
+    """Ensure subtest's msg is a plain str, which execnet can serialize (pytest-dev/pytest-xdist#1161)."""
+    context = SubtestContext(msg=msg, kwargs={})
+    assert context.msg == expected
+    assert type(context.msg) is type(expected)
+
+
 def test_serialization_xdist(pytester: pytest.Pytester) -> None:  # pragma: no cover
     """Regression test for pytest-dev/pytest-xdist#1273."""
     pytest.importorskip("xdist")
@@ -1059,10 +1078,17 @@ def test_serialization_xdist(pytester: pytest.Pytester) -> None:  # pragma: no c
         class MyEnum(Enum):
             A = "A"
 
+        class MyStrEnum(str, Enum):
+            B = "B"
+
         def test(subtests):
             with subtests.test(a=MyEnum.A):
                 pass
 
+        def test_str_subclass_msg(subtests):
+            with subtests.test(msg=MyStrEnum.B):
+                pass
+
         class T(unittest.TestCase):
 
             def test(self):
@@ -1072,4 +1098,4 @@ def test(self):
     )
     pytester.syspathinsert()
     result = pytester.runpytest("-n1", "-pxdist.plugin")
-    result.assert_outcomes(passed=2)
+    result.assert_outcomes(passed=3)