feat: make SignalGroupDescriptor generic

src/psygnal/_group_descriptor.py

@@ -5,12 +5,14 @@
 import sys
 import warnings
 import weakref
+from collections.abc import Iterable
 from contextlib import suppress
 from typing import (
     TYPE_CHECKING,
     Any,
     Callable,
     ClassVar,
+    Generic,
     Literal,
     Optional,
     TypeVar,
@@ -39,6 +41,7 @@
 
 T = TypeVar("T", bound=type)
 S = TypeVar("S")
+GroupType = TypeVar("GroupType", bound=SignalGroup)
 
 
 _EQ_OPERATORS: dict[type, dict[str, EqOperator]] = {}
@@ -153,10 +156,10 @@ def _psygnal_relocate_info_(self, emission_info: EmissionInfo) -> EmissionInfo:
 
 def _build_dataclass_signal_group(
     cls: type,
-    signal_group_class: type[SignalGroup],
+    signal_group_class: type[GroupType],
     equality_operators: Iterable[tuple[str, EqOperator]] | None = None,
     signal_aliases: Mapping[str, str | None] | FieldAliasFunc | None = None,
-) -> type[SignalGroup]:
+) -> type[GroupType]:
     """Build a SignalGroup with events for each field in a dataclass.
 
     Parameters
@@ -424,7 +427,7 @@ def _setattr_and_emit_(self: object, name: str, value: Any) -> None:
     return _inner(super_setattr) if super_setattr else _inner
 
 
-class SignalGroupDescriptor:
+class SignalGroupDescriptor(Generic[GroupType]):
     """Create a [`psygnal.SignalGroup`][] on first instance attribute access.
 
     This descriptor is designed to be used as a class attribute on a dataclass-like
@@ -544,12 +547,12 @@ def __init__(
         warn_on_no_fields: bool = True,
         cache_on_instance: bool = True,
         patch_setattr: bool = True,
-        signal_group_class: type[SignalGroup] | None = None,
+        signal_group_class: type[GroupType] | None = None,
         collect_fields: bool = True,
         connect_child_events: bool = True,
         signal_aliases: Mapping[str, str | None] | FieldAliasFunc | None = None,
     ):
-        grp_cls = signal_group_class or SignalGroup
+        grp_cls = signal_group_class or cast("type[GroupType]", SignalGroup)
         if not (isinstance(grp_cls, type) and issubclass(grp_cls, SignalGroup)):
             raise TypeError(  # pragma: no cover
                 f"'signal_group_class' must be a subclass of SignalGroup, not {grp_cls}"
@@ -574,11 +577,11 @@ def __init__(
         self._patch_setattr = patch_setattr
         self._connect_child_events = connect_child_events
 
-        self._signal_group_class: type[SignalGroup] = grp_cls
+        self._signal_group_class: type[GroupType] = grp_cls
         self._collect_fields = collect_fields
         self._signal_aliases = signal_aliases
 
-        self._signal_groups: dict[int, type[SignalGroup]] = {}
+        self._signal_groups: dict[int, type[GroupType]] = {}
 
     def __set_name__(self, owner: type, name: str) -> None:
         """Called when this descriptor is added to class `owner` as attribute `name`."""
@@ -618,11 +621,11 @@ def _do_patch_setattr(self, owner: type, with_aliases: bool = True) -> None:
     def __get__(self, instance: None, owner: type) -> SignalGroupDescriptor: ...
 
     @overload
-    def __get__(self, instance: object, owner: type) -> SignalGroup: ...
+    def __get__(self, instance: object, owner: type) -> GroupType: ...
 
     def __get__(
         self, instance: object, owner: type
-    ) -> SignalGroup | SignalGroupDescriptor:
+    ) -> GroupType | SignalGroupDescriptor:
         """Return a SignalGroup instance for `instance`."""
         if instance is None:
             return self
@@ -652,15 +655,16 @@ def __get__(
                     lambda: connect_child_events(instance, recurse=True, _group=grp)
                 )
 
-        return self._instance_map[obj_id]
+        return cast("GroupType", self._instance_map[obj_id])
 
-    def _get_signal_group(self, owner: type) -> type[SignalGroup]:
+    def _get_signal_group(self, owner: type) -> type[GroupType]:
         type_id = id(owner)
         if type_id not in self._signal_groups:
             self._signal_groups[type_id] = self._create_group(owner)
         return self._signal_groups[type_id]
 
-    def _create_group(self, owner: type) -> type[SignalGroup]:
+    def _create_group(self, owner: type) -> type[GroupType]:
+        # Do not collect fields from owner class, copy the SignalGroup
         if not self._collect_fields:
             # Do not collect fields from owner class
             Group = copy.deepcopy(self._signal_group_class)