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

Author: nicoddemus

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:

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
@@ -958,20 +960,52 @@ 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, ""),
         keywords={},
         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)