Fix pickling of exceptions with kw_only attributes

BaseException.__reduce__ passes all attribute values as positional
args, which fails when some attrs are keyword-only. Add a custom
__reduce__ to exception classes that passes all init arguments as
keyword arguments via a helper function.

Fixes #734

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Fridayai700

src/attr/_make.py

@@ -634,6 +634,37 @@ def evolve(*args, **changes):
     return cls(**changes)
 
 
+def _reconstruct_exc(cls, kwargs):
+    """
+    Reconstruct an attrs exception from keyword arguments.
+
+    Used by pickle to properly handle keyword-only arguments.
+    """
+    return cls(**kwargs)
+
+
+def _make_exc_reduce(attrs):
+    """
+    Create a ``__reduce__`` for exception classes that properly handles
+    keyword-only arguments during pickling.
+
+    BaseException's default ``__reduce__`` passes all values as positional
+    args, which fails when some attrs are keyword-only.
+    """
+    init_attrs = tuple(a for a in attrs if a.init)
+
+    def __reduce__(self):
+        return (
+            _reconstruct_exc,
+            (
+                self.__class__,
+                {a.name: getattr(self, a.name) for a in init_attrs},
+            ),
+        )
+
+    return __reduce__
+
+
 class _ClassBuilder:
     """
     Iteratively build *one* class.
@@ -749,6 +780,9 @@ def __init__(
                 self._cls_dict["__setstate__"],
             ) = self._make_getstate_setstate()
 
+        if props.is_exception:
+            self._cls_dict["__reduce__"] = _make_exc_reduce(attrs)
+
         # tuples of script, globs, hook
         self._script_snippets: list[
             tuple[str, dict, Callable[[dict, dict], Any]]

tests/test_functional.py

@@ -107,6 +107,28 @@ class WithMetaSlots(metaclass=Meta):
 FromMakeClass = attr.make_class("FromMakeClass", ["x"])
 
 
+@attr.s(auto_exc=True, kw_only=True)
+class KwOnlyExc(Exception):
+    x = attr.ib()
+
+
+@attr.s(auto_exc=True, slots=True, kw_only=True)
+class KwOnlyExcSlots(Exception):
+    x = attr.ib()
+
+
+@attr.s(auto_exc=True)
+class MixedExc(Exception):
+    x = attr.ib()
+    y = attr.ib(kw_only=True)
+
+
+@attr.s(auto_exc=True, slots=True)
+class MixedExcSlots(Exception):
+    x = attr.ib()
+    y = attr.ib(kw_only=True)
+
+
 class TestFunctional:
     """
     Functional tests.
@@ -613,6 +635,40 @@ class FooError(Exception):
 
         FooError(1)
 
+    @pytest.mark.parametrize(
+        "cls",
+        [KwOnlyExc, KwOnlyExcSlots],
+    )
+    def test_auto_exc_kw_only_pickle(self, cls):
+        """
+        Exceptions with kw_only=True can be pickled and unpickled.
+
+        Regression test for #734.
+        """
+        exc = cls(x=42)
+        exc2 = pickle.loads(pickle.dumps(exc))
+
+        assert exc2.x == 42
+        assert isinstance(exc2, cls)
+
+    @pytest.mark.parametrize(
+        "cls",
+        [MixedExc, MixedExcSlots],
+    )
+    def test_auto_exc_mixed_kw_only_pickle(self, cls):
+        """
+        Exceptions with a mix of positional and kw_only attrs can be
+        pickled and unpickled.
+
+        Regression test for #734.
+        """
+        exc = cls(1, y=2)
+        exc2 = pickle.loads(pickle.dumps(exc))
+
+        assert exc2.x == 1
+        assert exc2.y == 2
+        assert isinstance(exc2, cls)
+
     def test_eq_only(self, slots, frozen):
         """
         Classes with order=False cannot be ordered.