Fix @Frozen exceptions to allow __traceback__ to be set. (#1081)

changelog.d/1081.change.md

@@ -0,0 +1 @@
+Fix frozen exception classes when raised within e.g. `contextlib.contextmanager`, which mutates their `__traceback__` attributes.

src/attr/_make.py

@@ -12,7 +12,7 @@
 # We need to import _compat itself in addition to the _compat members to avoid
 # having the thread-local in the globals here.
 from . import _compat, _config, setters
-from ._compat import PY310, PYPY, _AnnotationExtractor, set_closure_cell
+from ._compat import PY310, _AnnotationExtractor, set_closure_cell
 from .exceptions import (
     DefaultAlreadySetError,
     FrozenInstanceError,
@@ -582,28 +582,19 @@ def _transform_attrs(
     return _Attributes((AttrsClass(attrs), base_attrs, base_attr_map))
 
 
-if PYPY:
-
-    def _frozen_setattrs(self, name, value):
-        """
-        Attached to frozen classes as __setattr__.
-        """
-        if isinstance(self, BaseException) and name in (
-            "__cause__",
-            "__context__",
-        ):
-            BaseException.__setattr__(self, name, value)
-            return
-
-        raise FrozenInstanceError()
-
-else:
+def _frozen_setattrs(self, name, value):
+    """
+    Attached to frozen classes as __setattr__.
+    """
+    if isinstance(self, BaseException) and name in (
+        "__cause__",
+        "__context__",
+        "__traceback__",
+    ):
+        BaseException.__setattr__(self, name, value)
+        return
 
-    def _frozen_setattrs(self, name, value):
-        """
-        Attached to frozen classes as __setattr__.
-        """
-        raise FrozenInstanceError()
+    raise FrozenInstanceError()
 
 
 def _frozen_delattrs(self, name):

tests/test_next_gen.py

@@ -6,6 +6,7 @@
 
 import re
 
+from contextlib import contextmanager
 from functools import partial
 
 import pytest
@@ -312,6 +313,38 @@ class MyException(Exception):
         assert "foo" == ei.value.x
         assert ei.value.__cause__ is None
 
+    @pytest.mark.parametrize(
+        "decorator",
+        [
+            partial(_attr.s, frozen=True, slots=True, auto_exc=True),
+            attrs.frozen,
+            attrs.define,
+            attrs.mutable,
+        ],
+    )
+    def test_setting_traceback_on_exception(self, decorator):
+        """
+        contextlib.contextlib (re-)sets __traceback__ on raised exceptions.
+
+        Ensure that works, as well as if done explicitly
+        """
+
+        @decorator
+        class MyException(Exception):
+            pass
+
+        @contextmanager
+        def do_nothing():
+            yield
+
+        with do_nothing(), pytest.raises(MyException) as ei:
+            raise MyException()
+
+        assert isinstance(ei.value, MyException)
+
+        # this should not raise an exception either
+        ei.value.__traceback__ = ei.value.__traceback__
+
     def test_converts_and_validates_by_default(self):
         """
         If no on_setattr is set, assume setters.convert, setters.validate.