Ensure subtest's context kwargs are JSON serializable

changelog/13963.bugfix.rst

@@ -0,0 +1,3 @@
+Fixed subtests running with `pytest-xdist <https://github.com/pytest-dev/pytest>`__ when their contexts contain objects that are not JSON-serializable.
+
+Fixes `pytest-dev/pytest-xdist#1273 <https://github.com/pytest-dev/pytest-xdist/issues/1273>`__.

src/_pytest/subtests.py

@@ -11,6 +11,7 @@
 from contextlib import ExitStack
 from contextlib import nullcontext
 import dataclasses
+import pickle
 import time
 from types import TracebackType
 from typing import Any
@@ -62,11 +63,16 @@ class SubtestContext:
     kwargs: Mapping[str, Any]
 
     def _to_json(self) -> dict[str, Any]:
-        return dataclasses.asdict(self)
+        result = dataclasses.asdict(self)
+        # Use protocol 0 because it is human-readable and guaranteed to be not-binary.
+        protocol = 0
+        data = pickle.dumps(result["kwargs"], protocol=protocol)
+        result["kwargs"] = data.decode("UTF-8")
+        return result
 
     @classmethod
     def _from_json(cls, d: dict[str, Any]) -> Self:
-        return cls(msg=d["msg"], kwargs=d["kwargs"])
+        return cls(msg=d["msg"], kwargs=pickle.loads(d["kwargs"].encode("UTF-8")))
 
 
 @dataclasses.dataclass(init=False)

testing/test_subtests.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from enum import Enum
+import json
 import sys
 from typing import Literal
 
@@ -957,7 +959,14 @@ def test(subtests):
     )
 
 
+class MyEnum(Enum):
+    """Used in test_serialization, needs to be declared at the module level to be pickled."""
+
+    A = "A"
+
+
 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
 
@@ -968,10 +977,41 @@ 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
+    # Ensure the report is actually serializable to JSON.
+    _ = json.dumps(data)
     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=10, a=MyEnum.A)
+    )
+
+
+def test_serialization_xdist(pytester: pytest.Pytester) -> None:  # pragma: no cover
+    """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
+        """
+    )
+    pytester.syspathinsert()
+    result = pytester.runpytest("-n1", "-pxdist.plugin")
+    result.assert_outcomes(passed=2)