Fix pickling of exception classes with kw_only attributes

BaseException.__reduce__ returns (cls, self.args, state), passing
self.args as positional arguments to cls() on unpickle. When kw_only=True,
__init__ only accepts keyword-only arguments, causing unpickling to fail
with: TypeError: __init__() takes 1 positional argument but 2 were given

This fix adds a custom __reduce__ for exception classes that have
kw_only attrs. It uses a _rebuild_exc helper that creates the instance
via __new__ (bypassing __init__) and restores attributes directly,
then calls BaseException.__init__ to properly set self.args.

Fixes GH#734

Author: contributor

src/attr/_make.py

@@ -103,6 +103,31 @@ def __reduce__(self, _none_constructor=type(None), _args=()):  # noqa: B008
         return _none_constructor, _args
 
 
+def _rebuild_exc(cls, state):
+    """
+    Rebuild an exception instance without calling ``__init__``.
+
+    Used by ``__reduce__`` for exception classes with ``kw_only`` attributes,
+    where ``BaseException.__reduce__`` would pass positional args that
+    ``kw_only`` rejects.
+    """
+    obj = cls.__new__(cls)
+    for name, value in state.items():
+        try:
+            object.__setattr__(obj, name, value)
+        except AttributeError:
+            pass
+    # Restore BaseException.args which is set by __init__ via
+    # BaseException.__init__(self, val1, val2, ...).
+    init_values = tuple(
+        state[a.name]
+        for a in cls.__attrs_attrs__
+        if a.init and a.name in state
+    )
+    BaseException.__init__(obj, *init_values)
+    return obj
+
+
 def attrib(
     default=NOTHING,
     validator=None,
@@ -757,6 +782,26 @@ def __init__(
                 self._cls_dict["__setstate__"],
             ) = self._make_getstate_setstate()
 
+        # Fix pickling for exception classes with kw_only attributes.
+        # BaseException.__reduce__ returns (cls, self.args, state) which calls
+        # cls(*args) on unpickle, but kw_only attrs reject positional args.
+        # Override __reduce__ to use __new__ + state instead of __init__.
+        if props.is_exception and any(
+            a.kw_only for a in attrs if a.init
+        ):
+            _attr_names = self._attr_names
+
+            def __reduce__(self, *, _attr_names=_attr_names):
+                state = {
+                    name: getattr(self, name)
+                    for name in _attr_names
+                    if name != "__weakref__"
+                    and hasattr(self, name)
+                }
+                return (_rebuild_exc, (self.__class__, state))
+
+            self._cls_dict["__reduce__"] = __reduce__
+
         # tuples of script, globs, hook
         self._script_snippets: list[
             tuple[str, dict, Callable[[dict, dict], Any]]

tests/test_functional.py

@@ -624,6 +624,45 @@ class FooError(Exception):
 
         FooError(1)
 
+    @pytest.mark.parametrize("protocol", range(pickle.HIGHEST_PROTOCOL + 1))
+    def test_auto_exc_kw_only_pickle(self, slots, frozen, protocol):
+        """
+        Exception classes with kw_only attributes can be pickled.
+
+        BaseException.__reduce__ passes positional args, which kw_only
+        rejects. Our custom __reduce__ handles this correctly.
+        Regression test for GH#734.
+        """
+
+        @attr.s(auto_exc=True, slots=slots, frozen=frozen, kw_only=True)
+        class KwOnlyError(Exception):
+            msg = attr.ib()
+            x = attr.ib()
+
+        e = KwOnlyError(msg="hello", x=42)
+        e2 = pickle.loads(pickle.dumps(e, protocol))
+
+        assert e2.msg == "hello"
+        assert e2.x == 42
+        assert e2.args == e.args
+
+    def test_auto_exc_mixed_kw_only_pickle(self, slots):
+        """
+        Exception classes with mixed positional and kw_only attributes
+        can be pickled.
+        """
+
+        @attr.s(auto_exc=True, slots=slots, kw_only=False)
+        class MixedError(Exception):
+            pos = attr.ib()
+            kw = attr.ib(kw_only=True)
+
+        e = MixedError(10, kw=20)
+        e2 = pickle.loads(pickle.dumps(e))
+
+        assert e2.pos == 10
+        assert e2.kw == 20
+
     def test_eq_only(self, slots, frozen):
         """
         Classes with order=False cannot be ordered.