closes: 10865 Fix muted exception

AUTHORS

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

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

@@ -329,3 +329,14 @@ def found_str():
                         module=w.__module__,
                         source=w.source,
                     )
+            # Check warnings has valid argument type (#10865).
+            wrn: warnings.WarningMessage
+            for wrn in self:
+                self._validate_message(wrn)
+
+    @staticmethod
+    def _validate_message(wrn: Any) -> None:
+        if not isinstance(msg := wrn.message.args[0], str):
+            raise TypeError(
+                f"Warning message must be str, got {msg!r} (type {type(msg).__name__})"
+            )

testing/test_recwarn.py

@@ -1,4 +1,5 @@
 # mypy: allow-untyped-defs
+import sys
 from typing import List
 from typing import Optional
 from typing import Type
@@ -477,3 +478,29 @@ def test_catch_warning_within_raise(self) -> None:
             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
+
+
+def test_no_raise_type_error_on_string_warning() -> None:
+    """Check pytest.warns validates warning messages are strings (#10865)."""
+    with pytest.warns(UserWarning):
+        warnings.warn("Warning")
+
+
+@pytest.mark.skipif(
+    hasattr(sys, "pypy_version_info"),
+    reason="Not for pypy",
+)
+def test_raise_type_error_on_non_string_warning_cpython() -> None:
+    # 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