[commands] add CustomDefault.converters to allow implicit conversions

[khazhyk: rebased on latest and resolved conflicts in _get_converter]

Author: sgtlaggy

discord/ext/commands/core.py

@@ -633,8 +633,10 @@ async def do_conversion(self, ctx, converter, argument, param):
     def _get_converter(self, param):
         converter = param.annotation
         if converter is param.empty:
-            if param.default is param.empty or param.default is None or (inspect.isclass(param.default) and issubclass(param.default, defaults.CustomDefault)) or isinstance(param.default, defaults.CustomDefault):
+            if param.default is param.empty or param.default is None:
                 converter = str
+            elif (inspect.isclass(param.default) and issubclass(param.default, defaults.CustomDefault)) or isinstance(param.default, defaults.CustomDefault):
+                converter = typing.Union[param.default.converters]
             else:
                 converter = type(param.default)
         return converter

discord/ext/commands/default.py

@@ -24,6 +24,8 @@
 DEALINGS IN THE SOFTWARE.
 """
 
+import discord
+
 from .errors import MissingRequiredArgument
 
 __all__ = (
@@ -49,9 +51,11 @@ def __str__(cls):
 class CustomDefault(metaclass=CustomDefaultMeta):
     """The base class of custom defaults that require the :class:`.Context`.
 
-    Classes that derive from this should override the :meth:`~.CustomDefault.default`
-    method to do its conversion logic. This method must be a coroutine.
+    Classes that derive from this should override the :attr:`~.CustomDefault.converters` attribute to specify
+    converters to use and the :meth:`~.CustomDefault.default` method to do its conversion logic.
+    This method must be a coroutine.
     """
+    converters = (str,)
 
     async def default(self, ctx, param):
         """|coro|
@@ -72,12 +76,14 @@ async def default(self, ctx, param):
 
 class Author(CustomDefault):
     """Default parameter which returns the author for this context."""
+    converters = (discord.Member, discord.User)
 
     async def default(self, ctx, param):
         return ctx.author
 
 class CurrentChannel(CustomDefault):
     """Default parameter which returns the channel for this context."""
+    converters = (discord.TextChannel,)
 
     async def default(self, ctx, param):
         return ctx.channel

docs/ext/commands/commands.rst

@@ -656,6 +656,7 @@ A DefaultParam returning ``None`` is valid - if this should be an error, raise :
 
     class LastImage(CustomDefault):
         """Default param which finds the last image in chat."""
+        converters = (AnyImage,)
 
         async def default(self, ctx, param):
             for attachment in message.attachments:
@@ -672,7 +673,7 @@ A DefaultParam returning ``None`` is valid - if this should be an error, raise :
             raise errors.MissingRequiredArgument(param)
 
     @bot.command()
-    async def echo_image(ctx, *, image: Image = LastImage):
+    async def echo_image(ctx, *, image=LastImage):
         async with aiohttp.ClientSession() as sess:
             async with sess.get(image) as resp:
                 resp.raise_for_status()