fix: make MagicFactory work as a method descriptor

src/magicgui/type_map/_magicgui.py

@@ -104,6 +104,37 @@ def __call__(self, *args: Any, **kwargs: Any) -> _FGuiVar:
             self._widget_init(widget)
         return widget
 
+    def __get__(self, obj: object, objtype: type | None = None) -> MagicFactory:
+        """Provide descriptor protocol.
+
+        This allows the `@magic_factory` decorator to work on a method as well as
+        on a plain function.  Accessing the attribute on an instance returns a
+        factory whose widgets bind the first parameter of the function to that
+        instance, mirroring `FunctionGui.__get__`.
+
+        Without this, `functools.partial.__get__` (added in Python 3.13) would
+        bind the instance as the first *positional* argument of the factory,
+        while on older versions the instance was simply never passed at all.
+        """
+        if obj is None:
+            return self
+        function = self.keywords.get("function")
+        if function is None:  # pragma: no cover
+            return self
+        p0 = next(iter(inspect.signature(function).parameters), None)
+        if p0 is None:  # pragma: no cover
+            return self
+        keywords = self.keywords.copy()
+        param_options = dict(keywords.pop("param_options", None) or {})
+        param_options.setdefault(p0, {"bind": obj})
+        return type(self)(
+            magic_class=self.func,  # type: ignore
+            widget_init=self._widget_init,
+            type_map=self._type_map,
+            param_options=param_options,
+            **keywords,
+        )
+
     def __getattr__(self, name: str) -> Any:
         """Allow accessing FunctionGui attributes without mypy error."""
 

tests/test_factory.py

@@ -145,3 +145,20 @@ def factory(arg=None):
         return 1
 
     assert factory()() == 1
+
+
+def test_magic_factory_as_method():
+    """A magic_factory used on a method should bind the instance to `self`."""
+
+    class MyClass:
+        def __init__(self, x: int):
+            self.x = x
+
+        @magic_factory
+        def factory(self):
+            return self.x
+
+    obj = MyClass(x=5)
+    widget = obj.factory()
+    assert isinstance(widget, FunctionGui)
+    assert widget() == 5