Work around CPython bug in HTTPError (#58)

CHANGES.rst

@@ -3,6 +3,12 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+
+- Workaround for [CPython issue #98778](https://github.com/python/cpython/issues/98778),
+  ``urllib.error.HTTPError(..., fp=None)`` raises ``KeyError`` on unknown attribute access,
+  on affected Python versions.  (PR by Zac Hatfield-Dodds)
+
 **1.1.0**
 
 - Backported upstream fix for gh-99553 (custom subclasses of ``BaseExceptionGroup`` that

src/exceptiongroup/_formatting.py

@@ -103,7 +103,16 @@ def __init__(
         # Capture now to permit freeing resources: only complication is in the
         # unofficial API _format_final_exc_line
         self._str = _safe_string(exc_value, "exception")
-        self.__notes__ = getattr(exc_value, "__notes__", None)
+        try:
+            self.__notes__ = getattr(exc_value, "__notes__", None)
+        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_value, HTTPError):
+                self.__notes__ = None
+            else:
+                raise
 
         if exc_type and issubclass(exc_type, SyntaxError):
             # Handle SyntaxError's specially

tests/test_formatting.py

@@ -1,5 +1,7 @@
 import sys
+import traceback
 from typing import NoReturn
+from urllib.error import HTTPError
 
 import pytest
 from _pytest.capture import CaptureFixture
@@ -528,3 +530,9 @@ def __init__(self, name: str) -> None:
         print_exception(e)  # does not crash
         output = capsys.readouterr().err
         assert "NamedAttributeError" in output
+
+
+def test_works_around_httperror_bug():
+    # See https://github.com/python/cpython/issues/98778 in Python <= 3.9
+    err = HTTPError("url", 405, "METHOD NOT ALLOWED", None, None)
+    traceback.TracebackException(type(err), err, None)