From 7585f4500ab8081454151563743e1983e4f51f8e Mon Sep 17 00:00:00 2001
From: Bruno Oliveira <bruno@pytest.org>
Date: Thu, 13 Nov 2025 19:27:26 -0300
Subject: [PATCH] Ensure subtest's context kwargs are JSON serializable

Convert all the values of `SubtestContext.kwargs` to strings using `saferepr`.

This complies with the requirement that the returned dict from `pytest_report_to_serializable` is serializable to JSON, at the cost of losing type information for objects that are natively supported by JSON.

Fixes pytest-dev/pytest-xdist#1273
---
 changelog/13963.bugfix.rst |  3 +++
 src/_pytest/subtests.py    |  5 ++++-
 testing/test_subtests.py   | 38 ++++++++++++++++++++++++++++++++++++--
 3 files changed, 43 insertions(+), 3 deletions(-)
 create mode 100644 changelog/13963.bugfix.rst

diff --git a/changelog/13963.bugfix.rst b/changelog/13963.bugfix.rst
new file mode 100644
index 00000000000..90cb5ae6315
--- /dev/null
+++ b/changelog/13963.bugfix.rst
@@ -0,0 +1,3 @@
+Fixed subtests running with `pytest-xdist <https://github.com/pytest-dev/pytest>`__ when their contexts contain non-standard objects.
+
+Fixes `pytest-dev/pytest-xdist#1273 <https://github.com/pytest-dev/pytest-xdist/issues/1273>`__.
diff --git a/src/_pytest/subtests.py b/src/_pytest/subtests.py
index e0ceb27f4b1..d00346e076d 100644
--- a/src/_pytest/subtests.py
+++ b/src/_pytest/subtests.py
@@ -62,7 +62,10 @@ class SubtestContext:
     kwargs: Mapping[str, Any]
 
     def _to_json(self) -> dict[str, Any]:
-        return dataclasses.asdict(self)
+        result = dataclasses.asdict(self)
+        # Brute-force the returned kwargs dict to be JSON serializable (pytest-dev/pytest-xdist#1273).
+        result["kwargs"] = {k: saferepr(v) for (k, v) in result["kwargs"].items()}
+        return result
 
     @classmethod
     def _from_json(cls, d: dict[str, Any]) -> Self:
diff --git a/testing/test_subtests.py b/testing/test_subtests.py
index 6849df53622..8af48d00d02 100644
--- a/testing/test_subtests.py
+++ b/testing/test_subtests.py
@@ -1,8 +1,10 @@
 from __future__ import annotations
 
+from enum import Enum
 import sys
 from typing import Literal
 
+from _pytest._io.saferepr import saferepr
 from _pytest.subtests import SubtestContext
 from _pytest.subtests import SubtestReport
 import pytest
@@ -958,9 +960,13 @@ def test(subtests):
 
 
 def test_serialization() -> None:
+    """Ensure subtest's kwargs are serialized using `saferepr` (pytest-dev/pytest-xdist#1273)."""
     from _pytest.subtests import pytest_report_from_serializable
     from _pytest.subtests import pytest_report_to_serializable
 
+    class MyEnum(Enum):
+        A = "A"
+
     report = SubtestReport(
         "test_foo::test_foo",
         ("test_foo.py", 12, ""),
@@ -968,10 +974,38 @@ def test_serialization() -> None:
         outcome="passed",
         when="call",
         longrepr=None,
-        context=SubtestContext(msg="custom message", kwargs=dict(i=10)),
+        context=SubtestContext(msg="custom message", kwargs=dict(i=10, a=MyEnum.A)),
     )
     data = pytest_report_to_serializable(report)
     assert data is not None
     new_report = pytest_report_from_serializable(data)
     assert new_report is not None
-    assert new_report.context == SubtestContext(msg="custom message", kwargs=dict(i=10))
+    assert new_report.context == SubtestContext(
+        msg="custom message", kwargs=dict(i=saferepr(10), a=saferepr(MyEnum.A))
+    )
+
+
+def test_serialization_xdist(pytester: pytest.Pytester) -> None:
+    """Regression test for pytest-dev/pytest-xdist#1273."""
+    pytest.importorskip("xdist")
+    pytester.makepyfile(
+        """
+        from enum import Enum
+        import unittest
+
+        class MyEnum(Enum):
+            A = "A"
+
+        def test(subtests):
+            with subtests.test(a=MyEnum.A):
+                pass
+
+        class T(unittest.TestCase):
+
+            def test(self):
+                with self.subTest(a=MyEnum.A):
+                    pass
+        """
+    )
+    result = pytester.runpytest("-n1", "-pxdist.plugin")
+    result.assert_outcomes(passed=2)