feat: 10865 refactor code and tests

AUTHORS

@@ -416,6 +416,7 @@ Vivaan Verma
 Vlad Dragos
 Vlad Radziuk
 Vladyslav Rachek
+Volodymyr Kochetkov
 Volodymyr Piskun
 Wei Lin
 Wil Cooley

changelog/10865.bugfix.rst

@@ -1 +0,0 @@
-Fix a TypeError in a warning arguments call muted by warnings filter.

changelog/10865.improvement.rst

@@ -0,0 +1,2 @@
+:func:`pytest.warns` now validates that warning object's ``message`` is of type  `str` -- currently in Python it is possible to pass other types than `str` when creating `Warning` instances, however this causes an exception when :func:`warnings.filterwarnings` is used to filter those warnings. See `CPython #103577 <https://github.com/python/cpython/issues/103577>`__ for a discussion.
+While this can be considered a bug in CPython, we decided to put guards in pytest as the error message produced without this check in place is confusing.

src/_pytest/recwarn.py

@@ -329,11 +329,11 @@ class WarningsChecker(WarningsRecorder):
                         module=w.__module__,
                         source=w.source,
                     )
-            # Check warnings has valid argument type
+            # Check warnings has valid argument type (#10865).
             wrn: warnings.WarningMessage
             for wrn in self:
                 if isinstance(wrn.message, Warning):
-                    if not isinstance(wrn.message.args[0], str):
+                    if not isinstance(msg := wrn.message.args[0], str):
                         raise TypeError(
-                            f"Warning message must be str, got {type(wrn.message.args[0])}"
+                            f"Warning message must be str, got {msg!r} (type {type(msg).__name__})"
                         )

testing/python/test_warning_attributes.py

@@ -1,17 +0,0 @@
-from _pytest.pytester import Pytester
-
-
-class TestWarningAttributes:
-    def test_raise_type_error(self, pytester: Pytester) -> None:
-        pytester.makepyfile(
-            """
-            import pytest
-            import warnings
-
-            def test_example_one():
-                with pytest.warns(UserWarning):
-                    warnings.warn(1)
-            """
-        )
-        result = pytester.runpytest()
-        result.stdout.fnmatch_lines(["*1 failed*"])

testing/test_recwarn.py

@@ -477,3 +477,17 @@ class TestWarns:
             with pytest.raises(ValueError, match="some exception"):
                 warnings.warn("some warning", category=FutureWarning)
                 raise ValueError("some exception")
+
+
+def test_raise_type_error_on_non_string_warning() -> None:
+    """Check pytest.warns validates warning messages are strings (#10865)."""
+    with pytest.raises(TypeError, match="Warning message must be str"):
+        with pytest.warns(UserWarning):
+            warnings.warn(1)  # type: ignore
+
+    # Check that we get the same behavior with the stdlib, at least if filtering
+    # (see https://github.com/python/cpython/issues/103577 for details)
+    with pytest.raises(TypeError):
+        with warnings.catch_warnings():
+            warnings.filterwarnings("ignore", "test")
+            warnings.warn(1)  # type: ignore