Best-effort to convert to JSON

Author: nicoddemus

changelog/13963.bugfix.rst

@@ -1,3 +1,3 @@
-Fixed subtests running with `pytest-xdist <https://github.com/pytest-dev/pytest>`__ when their contexts contain non-standard objects.
+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 json
 import time
 from types import TracebackType
 from typing import Any
@@ -63,8 +64,19 @@ class SubtestContext:
 
     def _to_json(self) -> dict[str, Any]:
         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()}
+
+        # Best-effort to convert the kwargs values to JSON (pytest-dev/pytest-xdist#1273).
+        # If they can be converted, we return as it is, otherwise we return its saferepr because it seems
+        # this is the best we can do at this point.
+        def convert(x: Any) -> Any:
+            try:
+                json.dumps(x)
+            except TypeError:
+                return saferepr(x)
+            else:
+                return x
+
+        result["kwargs"] = {k: convert(v) for (k, v) in result["kwargs"].items()}
         return result
 
     @classmethod

testing/test_subtests.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from enum import Enum
+import json
 import sys
 from typing import Literal
 
@@ -978,14 +979,16 @@ class MyEnum(Enum):
     )
     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=saferepr(10), a=saferepr(MyEnum.A))
+        msg="custom message", kwargs=dict(i=10, a=saferepr(MyEnum.A))
     )
 
 
-def test_serialization_xdist(pytester: pytest.Pytester) -> None:
+def test_serialization_xdist(pytester: pytest.Pytester) -> None:  # pragma: no cover
     """Regression test for pytest-dev/pytest-xdist#1273."""
     pytest.importorskip("xdist")
     pytester.makepyfile(