Add autocomplete method to Transformers

Co-authored-by: Danny <Rapptz@users.noreply.github.com>

discord/app_commands/commands.py

@@ -167,12 +167,20 @@ def validate_context_menu_name(name: str) -> str:
 
 
 def _validate_auto_complete_callback(
-    callback: AutocompleteCallback[GroupT, ChoiceT]
+    callback: AutocompleteCallback[GroupT, ChoiceT],
+    skip_binding: bool = False,
 ) -> AutocompleteCallback[GroupT, ChoiceT]:
 
-    requires_binding = is_inside_class(callback)
+    binding = getattr(callback, '__self__', None)
-    required_parameters = 2 + requires_binding
+    if binding is not None:
+        callback = callback.__func__
+
+    requires_binding = (binding is None and is_inside_class(callback)) or skip_binding
+
     callback.requires_binding = requires_binding
+    callback.binding = binding
+
+    required_parameters = 2 + requires_binding
     params = inspect.signature(callback).parameters
     if len(params) != required_parameters:
         raise TypeError('autocomplete callback requires either 2 or 3 parameters to be passed')
@@ -581,8 +589,9 @@ class Command(Generic[GroupT, P, T]):
             raise CommandSignatureMismatch(self)
 
         if param.autocomplete.requires_binding:
-            if self.binding is not None:
+            binding = param.autocomplete.binding or self.binding
-                choices = await param.autocomplete(self.binding, interaction, value)
+            if binding is not None:
+                choices = await param.autocomplete(binding, interaction, value)
             else:
                 raise TypeError('autocomplete parameter expected a bound self parameter but one was not provided')
         else:

discord/app_commands/transformers.py

@@ -61,6 +61,8 @@ __all__ = (
 )
 
 T = TypeVar('T')
+FuncT = TypeVar('FuncT', bound=Callable[..., Any])
+ChoiceT = TypeVar('ChoiceT', str, int, float, Union[str, int, float])
 NoneType = type(None)
 
 if TYPE_CHECKING:
@@ -249,6 +251,35 @@ class Transformer:
         """
         raise NotImplementedError('Derived classes need to implement this.')
 
+    @classmethod
+    async def autocomplete(
+        cls, interaction: Interaction, value: Union[int, float, str]
+    ) -> List[Choice[Union[int, float, str]]]:
+        """|coro|
+
+        An autocomplete prompt handler to be automatically used by options using this transformer.
+
+        .. note::
+
+            Autocomplete is only supported for options with a :meth:`~discord.app_commands.Transformer.type`
+            of :attr:`~discord.AppCommandOptionType.string`, :attr:`~discord.AppCommandOptionType.integer`,
+            or :attr:`~discord.AppCommandOptionType.number`.
+
+        Parameters
+        -----------
+        interaction: :class:`~discord.Interaction`
+            The autocomplete interaction being handled.
+        value: Union[:class:`str`, :class:`int`, :class:`float`]
+            The current value entered by the user.
+
+        Returns
+        --------
+        List[:class:`~discord.app_commands.Choice`]
+            A list of choices to be displayed to the user, a maximum of 25.
+
+        """
+        raise NotImplementedError('Derived classes can implement this.')
+
 
 class _TransformMetadata:
     __discord_app_commands_transform__: ClassVar[bool] = True
@@ -679,4 +710,9 @@ def annotation_to_parameter(annotation: Any, parameter: inspect.Parameter) -> Co
     if parameter.kind in (parameter.POSITIONAL_ONLY, parameter.VAR_KEYWORD, parameter.VAR_POSITIONAL):
         raise TypeError(f'unsupported parameter kind in callback: {parameter.kind!s}')
 
+    if inner.autocomplete is not Transformer.autocomplete:
+        from .commands import _validate_auto_complete_callback
+
+        result.autocomplete = _validate_auto_complete_callback(inner.autocomplete, skip_binding=True)
+
     return result