BUG: fix an edge case where ExceptionInfo._stringify_exception could crash pytest.raises

AUTHORS

@@ -93,6 +93,7 @@ Christopher Dignam
 Christopher Gilling
 Claire Cecil
 Claudio Madotto
+Clément M.T. Robert
 CrazyMerlyn
 Cristian Vera
 Cyrus Maden

changelog/11879.bugfix.rst

@@ -0,0 +1 @@
+Fix an edge case where ``ExceptionInfo._stringify_exception`` could crash :func:`pytest.raises`.

src/_pytest/_code/code.py

@@ -699,10 +699,21 @@
         return fmt.repr_excinfo(self)
 
     def _stringify_exception(self, exc: BaseException) -> str:
+        try:
+            notes = getattr(exc, "__notes__", [])
+        except KeyError:
+            # Workaround for https://github.com/python/cpython/issues/98778 on
+            # Python <= 3.9, and some 3.10 and 3.11 patch versions.
+            HTTPError = getattr(sys.modules.get("urllib.error", None), "HTTPError", ())
+            if sys.version_info[:2] <= (3, 11) and isinstance(exc, HTTPError):
+                notes = []
+            else:
+                raise
+
         return "\n".join(
             [
                 str(exc),
-                *getattr(exc, "__notes__", []),
+                *notes,
             ]
         )
 

testing/python/raises.py

@@ -302,3 +302,16 @@ class NotAnException:
             with pytest.raises(("hello", NotAnException)):  # type: ignore[arg-type]
                 pass  # pragma: no cover
         assert "must be a BaseException type, not str" in str(excinfo.value)
+
+    def test_issue_11872(self) -> None:
+        """Regression test for #11872.
+
+        urllib.error.HTTPError on Python<=3.9 raises KeyError instead of
+        AttributeError on invalid attribute access.
+
+        https://github.com/python/cpython/issues/98778
+        """
+        from urllib.error import HTTPError
+
+        with pytest.raises(HTTPError, match="Not Found"):
+            raise HTTPError(code=404, msg="Not Found", fp=None, hdrs=None, url="")  # type: ignore [arg-type]