fix: fix connect_setattr on dataclass field signals (#258)

* fix: fix connect_setattr on dataclass field signals

* add comment

* fix lint

src/psygnal/_group_descriptor.py

@@ -20,12 +20,13 @@
 
 from ._dataclass_utils import iter_fields
 from ._group import SignalGroup
-from ._signal import Signal
+from ._signal import Signal, SignalInstance
 
 if TYPE_CHECKING:
+    from _weakref import ref as ref
     from typing_extensions import Literal
 
-    from ._signal import SignalInstance
+    from psygnal._weak_callback import RefErrorChoice, WeakCallback
 
 
 __all__ = ["is_evented", "get_evented_namespace", "SignalGroupDescriptor"]
@@ -125,6 +126,18 @@ def _pick_equality_operator(type_: type | None) -> EqOperator:
     return operator.eq
 
 
+class _DataclassFieldSignalInstance(SignalInstance):
+    def connect_setattr(
+        self,
+        obj: ref | object,
+        attr: str,
+        maxargs: int | None = 1,
+        *,
+        on_ref_error: RefErrorChoice = "warn",
+    ) -> WeakCallback[None]:
+        return super().connect_setattr(obj, attr, maxargs, on_ref_error=on_ref_error)
+
+
 @lru_cache(maxsize=None)
 def _build_dataclass_signal_group(
     cls: type, equality_operators: Iterable[tuple[str, EqOperator]] | None = None
@@ -133,14 +146,18 @@ def _build_dataclass_signal_group(
     _equality_operators = dict(equality_operators) if equality_operators else {}
     signals = {}
     eq_map = _get_eq_operator_map(cls)
+    # create a Signal for each field in the dataclass
     for name, type_ in iter_fields(cls):
         if name in _equality_operators:
             if not callable(_equality_operators[name]):  # pragma: no cover
                 raise TypeError("EqOperator must be callable")
             eq_map[name] = _equality_operators[name]
         else:
             eq_map[name] = _pick_equality_operator(type_)
-        signals[name] = Signal(object if type_ is None else type_)
+        field_type = object if type_ is None else type_
+        signals[name] = sig = Signal(field_type)
+        # patch in our custom SignalInstance class with maxargs=1 on connect_setattr
+        sig._signal_instance_class = _DataclassFieldSignalInstance
 
     return type(f"{cls.__name__}SignalGroup", (SignalGroup,), signals)
 
@@ -380,7 +397,8 @@ def _do_patch_setattr(self, owner: type) -> None:
         try:
             # assign a new __setattr__ method to the class
             owner.__setattr__ = evented_setattr(  # type: ignore
-                self._name, owner.__setattr__  # type: ignore
+                cast(str, self._name),
+                owner.__setattr__,  # type: ignore
             )
         except Exception as e:  # pragma: no cover
             # not sure what might cause this ... but it will have consequences

src/psygnal/_signal.py

@@ -116,6 +116,7 @@ def __init__(
         self.description = description
         self._check_nargs_on_connect = check_nargs_on_connect
         self._check_types_on_connect = check_types_on_connect
+        self._signal_instance_class: type[SignalInstance] = SignalInstance
 
         if types and isinstance(types[0], Signature):
             self._signature = types[0]
@@ -171,7 +172,7 @@ class Emitter:
         if instance is None:
             return self
         name = cast("str", self._name)
-        signal_instance = SignalInstance(
+        signal_instance = self._signal_instance_class(
             self.signature,
             instance=instance,
             name=name,

tests/test_group_descriptor.py

@@ -186,3 +186,29 @@ def b(self, value: int) -> None:
     b_mock.assert_not_called()  # getter shouldn't have been called
     assert foo.b == 1
     b_mock.assert_called_once_with(1)  # getter should have been called only once
+
+
+def test_evented_field_connect_setattr() -> None:
+    """Test that using connect_setattr"""
+
+    @dataclass
+    class Foo:
+        a: int
+        events: ClassVar = SignalGroupDescriptor()
+
+    class Bar:
+        x = 1
+        y = 1
+
+    foo = Foo(a=1)
+    bar = Bar()
+
+    foo.events.a.connect_setattr(bar, "x")
+    foo.events.a.connect_setattr(bar, "y", maxargs=None)
+    foo.events.a.emit(2, 1)
+
+    assert bar.x == 2  # this is likely the desired outcome
+    # this is a bit of a gotcha, but it's the expected behavior
+    # when using connect_setattr with maxargs=None
+    # remove this test if/when we change maxargs to default to 1 on SignalInstance
+    assert bar.y == (2, 1)  # type: ignore