[commands] Make Command a descriptor for #426.

discord/ext/commands/bot.py

@@ -417,7 +417,6 @@ class BotBase(GroupMixin):
         for name, member in members:
             # register commands the cog has
             if isinstance(member, Command):
-                member.instance = cog
                 if member.parent is None:
                     self.add_command(member)
                 continue
@@ -465,7 +464,6 @@ class BotBase(GroupMixin):
         for name, member in members:
             # remove commands the cog has
             if isinstance(member, Command):
-                member.instance = None
                 if member.parent is None:
                     self.remove_command(member.name)
                 continue

discord/ext/commands/core.py

@@ -160,6 +160,11 @@ class Command:
         finally:
             ctx.bot.dispatch('command_error', error, ctx)
 
+    def __get__(self, instance, owner):
+        if instance is not None:
+            self.instance = instance
+        return self
+
     @asyncio.coroutine
     def do_conversion(self, ctx, converter, argument):
         if converter is bool: