From 6fc8c29da0f1803c8707f6759e243104004beb08 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Mon, 5 Oct 2026 08:14:42 +0000
Subject: [PATCH] subtests: send str subclass messages as plain str

A str subclass passed as the subtest msg, such as a StrEnum member,
was stored on the report as-is. execnet serializes by exact type, so
pytest-xdist failed with "DumpError: can't serialize <enum ...>".

Convert such messages with str.__str__ in SubtestContext, next to the
existing saferepr handling of kwargs. str.__str__ keeps the string data;
str() on a (str, Enum) mixin would give "Cls.MEMBER".

Refs pytest-dev/pytest-xdist#1161.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014AXM2385U5x4VaR1ppeo4X
---
 changelog/15134.bugfix.rst |  1 +
 src/_pytest/subtests.py    |  6 ++++++
 testing/test_subtests.py   | 28 +++++++++++++++++++++++++++-
 3 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 changelog/15134.bugfix.rst

diff --git a/changelog/15134.bugfix.rst b/changelog/15134.bugfix.rst
new file mode 100644
index 00000000000..2a231460478
--- /dev/null
+++ b/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``.
diff --git a/src/_pytest/subtests.py b/src/_pytest/subtests.py
index 1a908a15e8f..ea0d9af6573 100644
--- a/src/_pytest/subtests.py
+++ b/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)
diff --git a/testing/test_subtests.py b/testing/test_subtests.py
index 877e32b5204..822d6f631b7 100644
--- a/testing/test_subtests.py
+++ b/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)