allow __suppress_context__ and __notes__ to be mutated on frozen exceptions (#1365)

* allow __suppress_context__ to be mutated on frozen exceptions

* add changelog

* and __notes__! lets' not forget that.

* add notes to news

* add tests

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix improt

* fix test

* fix tests again

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

changelog.d/1365.change.md

@@ -0,0 +1 @@
+Allow mutating `__suppress_context__` and `__notes__` on frozen exceptions.

src/attr/_make.py

@@ -543,6 +543,8 @@ def _frozen_setattrs(self, name, value):
         "__cause__",
         "__context__",
         "__traceback__",
+        "__suppress_context__",
+        "__notes__",
     ):
         BaseException.__setattr__(self, name, value)
         return
@@ -554,6 +556,10 @@ def _frozen_delattrs(self, name):
     """
     Attached to frozen classes as __delattr__.
     """
+    if isinstance(self, BaseException) and name in ("__notes__",):
+        BaseException.__delattr__(self, name)
+        return
+
     raise FrozenInstanceError
 
 

tests/test_next_gen.py

@@ -14,6 +14,8 @@ import pytest
 import attr as _attr  # don't use it by accident
 import attrs
 
+from attr._compat import PY_3_11_PLUS
+
 
 @attrs.define
 class C:
@@ -332,7 +334,7 @@ class TestNextGen:
             attrs.mutable,
         ],
     )
-    def test_setting_traceback_on_exception(self, decorator):
+    def test_setting_exception_mutable_attributes(self, decorator):
         """
         contextlib.contextlib (re-)sets __traceback__ on raised exceptions.
 
@@ -354,6 +356,16 @@ class TestNextGen:
 
         # this should not raise an exception either
         ei.value.__traceback__ = ei.value.__traceback__
+        ei.value.__cause__ = ValueError("cause")
+        ei.value.__context__ = TypeError("context")
+        ei.value.__suppress_context__ = True
+        ei.value.__suppress_context__ = False
+        ei.value.__notes__ = []
+        del ei.value.__notes__
+
+        if PY_3_11_PLUS:
+            ei.value.add_note("note")
+            del ei.value.__notes__
 
     def test_converts_and_validates_by_default(self):
         """