fix: bind magic factories as method descriptors

src/magicgui/type_map/_magicgui.py

@@ -2,7 +2,7 @@
 
 import inspect
 from functools import partial
-from typing import Any, Callable, Generic, TypeVar
+from typing import Any, Callable, Generic, TypeVar, cast
 
 from magicgui.type_map._type_map import TypeMap
 from magicgui.widgets import FunctionGui
@@ -83,6 +83,28 @@ def __repr__(self) -> str:
         ]
         return f"MagicFactory({', '.join(args)})"
 
+    def __get__(self, obj: Any, objtype: type | None = None) -> MagicFactory[_FGuiVar]:
+        """Bind the factory's function when accessed as a method descriptor."""
+        if obj is None:
+            return self
+
+        factory_kwargs = self.keywords.copy()
+        function = factory_kwargs.pop("function")
+        function_get = getattr(function, "__get__", None)
+        bound_function = (
+            function_get(obj, objtype)
+            if function_get is not None
+            else partial(function, obj)
+        )
+        return type(self)(
+            bound_function,
+            *self.args,
+            magic_class=cast("type[_FGuiVar]", self.func),
+            widget_init=self._widget_init,
+            type_map=self._type_map,
+            **factory_kwargs,
+        )
+
     # TODO: annotate args and kwargs here so that
     # calling a MagicFactory instance gives proper mypy hints
     def __call__(self, *args: Any, **kwargs: Any) -> _FGuiVar:

tests/test_factory.py

@@ -1,3 +1,6 @@
+import gc
+import weakref
+
 import pytest
 from psygnal import EmitLoopError
 
@@ -48,6 +51,50 @@ def factory(x="a"):
     assert isinstance(widget_b.x, ComboBox)
 
 
+def test_magic_factory_method_descriptor():
+    initialized = []
+
+    def widget_init(widget):
+        initialized.append(widget)
+
+    class Example:
+        def __init__(self, value):
+            self.value = value
+
+        @magic_factory(call_button=True, widget_init=widget_init)
+        def factory(self, suffix: str = ""):
+            return f"{self.value}{suffix}"
+
+    first = Example("first")
+    second = Example("second")
+    first_widget = first.factory()
+    second_widget = second.factory()
+
+    assert Example.factory is Example.__dict__["factory"]
+    assert first_widget.call_button
+    assert first_widget() == "first"
+    assert second_widget(suffix="!") == "second!"
+    assert initialized == [first_widget, second_widget]
+
+
+def test_magic_factory_method_descriptor_does_not_retain_instance():
+    class Example:
+        @magic_factory
+        def factory(self):
+            return self
+
+    instance = Example()
+    instance_ref = weakref.ref(instance)
+    bound_factory = instance.factory
+
+    del instance
+    assert instance_ref() is not None
+
+    del bound_factory
+    gc.collect()
+    assert instance_ref() is None
+
+
 def test_magic_factory_repr():
     """Test basic magic_factory behavior."""