[commands] Converter.convert is always a coroutine.

Along with this change comes with the removal of Converter.prepare and
adding two arguments to Converter.convert, the context and the argument.

I suppose an added benefit is that you don't have to do attribute
access since it's a local variable.

2 files changed, 83 insertions, 78 deletions

diff --git a/discord/ext/commands/converter.py b/discord/ext/commands/converter.py
index c3d1c787..36e53fd2 100644
--- a/discord/ext/commands/converter.py
+++ b/discord/ext/commands/converter.py
@@ -53,46 +53,52 @@ class Converter:
     special cased ``discord`` classes.
 
     Classes that derive from this should override the :meth:`convert` method
-    to do its conversion logic. This method could be a coroutine or a regular
-    function.
-
-    Before the convert method is called, :meth:`prepare` is called. This
-    method must set the attributes below if overwritten.
-
-    Attributes
-    -----------
-    ctx: :class:`Context`
-        The invocation context that the argument is being used in.
-    argument: str
-        The argument that is being converted.
+    to do its conversion logic. This method must be a coroutine.
     """
-    def prepare(self, ctx, argument):
-        self.ctx = ctx
-        self.argument = argument
 
-    def convert(self):
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        """|coro|
+
+        The method to override to do conversion logic.
+
+        This can either be a coroutine or a regular function.
+
+        If an error is found while converting, it is recommended to
+        raise a :class:`CommandError` derived exception as it will
+        properly propagate to the error handlers.
+
+        Parameters
+        -----------
+        ctx: :class:`Context`
+            The invocation context that the argument is being used in.
+        argument: str
+            The argument that is being converted.
+        """
         raise NotImplementedError('Derived classes need to implement this.')
 
 class IDConverter(Converter):
     def __init__(self):
         self._id_regex = re.compile(r'([0-9]{15,21})$')
+        super().__init__()
 
-    def _get_id_match(self):
-        return self._id_regex.match(self.argument)
+    def _get_id_match(self, argument):
+        return self._id_regex.match(argument)
 
 class MemberConverter(IDConverter):
-    def convert(self):
-        message = self.ctx.message
-        bot = self.ctx.bot
-        match = self._get_id_match() or re.match(r'<@!?([0-9]+)>$', self.argument)
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        message = ctx.message
+        bot = ctx.bot
+        match = self._get_id_match(argument) or re.match(r'<@!?([0-9]+)>$', argument)
         guild = message.guild
         result = None
         if match is None:
             # not a mention...
             if guild:
-                result = guild.get_member_named(self.argument)
+                result = guild.get_member_named(argument)
             else:
-                result = _get_from_guilds(bot, 'get_member_named', self.argument)
+                result = _get_from_guilds(bot, 'get_member_named', argument)
         else:
             user_id = int(match.group(1))
             if guild:
@@ -101,21 +107,22 @@ class MemberConverter(IDConverter):
                 result = _get_from_guilds(bot, 'get_member', user_id)
 
         if result is None:
-            raise BadArgument('Member "{}" not found'.format(self.argument))
+            raise BadArgument('Member "{}" not found'.format(argument))
 
         return result
 
 class UserConverter(IDConverter):
-    def convert(self):
-        match = self._get_id_match() or re.match(r'<@!?([0-9]+)>$', self.argument)
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        match = self._get_id_match(argument) or re.match(r'<@!?([0-9]+)>$', argument)
         result = None
-        state = self.ctx._state
+        state = ctx._state
 
         if match is not None:
             user_id = int(match.group(1))
-            result = self.ctx.bot.get_user(user_id)
+            result = ctx.bot.get_user(user_id)
         else:
-            arg = self.argument
+            arg = argument
             # check for discriminator if it exists
             if len(arg) > 5 and arg[-5] == '#':
                 discrim = arg[-4:]
@@ -129,25 +136,26 @@ class UserConverter(IDConverter):
             result = discord.utils.find(predicate, state._users.values())
 
         if result is None:
-            raise BadArgument('User "{}" not found'.format(self.argument))
+            raise BadArgument('User "{}" not found'.format(argument))
 
         return result
 
 class TextChannelConverter(IDConverter):
-    def convert(self):
-        bot = self.ctx.bot
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        bot = ctx.bot
 
-        match = self._get_id_match() or re.match(r'<#([0-9]+)>$', self.argument)
+        match = self._get_id_match(argument) or re.match(r'<#([0-9]+)>$', argument)
         result = None
-        guild = self.ctx.guild
+        guild = ctx.guild
 
         if match is None:
             # not a mention
             if guild:
-                result = discord.utils.get(guild.text_channels, name=self.argument)
+                result = discord.utils.get(guild.text_channels, name=argument)
             else:
                 def check(c):
-                    return isinstance(c, discord.TextChannel) and c.name == self.argument
+                    return isinstance(c, discord.TextChannel) and c.name == argument
                 result = discord.utils.find(check, bot.get_all_channels())
         else:
             channel_id = int(match.group(1))
@@ -157,25 +165,25 @@ class TextChannelConverter(IDConverter):
                 result = _get_from_guilds(bot, 'get_channel', channel_id)
 
         if not isinstance(result, discord.TextChannel):
-            raise BadArgument('Channel "{}" not found.'.format(self.argument))
+            raise BadArgument('Channel "{}" not found.'.format(argument))
 
         return result
 
 class VoiceChannelConverter(IDConverter):
-    def convert(self):
-        bot = self.ctx.bot
-
-        match = self._get_id_match() or re.match(r'<#([0-9]+)>$', self.argument)
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        bot = ctx.bot
+        match = self._get_id_match(argument) or re.match(r'<#([0-9]+)>$', argument)
         result = None
-        guild = self.ctx.guild
+        guild = ctx.guild
 
         if match is None:
             # not a mention
             if guild:
-                result = discord.utils.get(guild.voice_channels, name=self.argument)
+                result = discord.utils.get(guild.voice_channels, name=argument)
             else:
                 def check(c):
-                    return isinstance(c, discord.VoiceChannel) and c.name == self.argument
+                    return isinstance(c, discord.VoiceChannel) and c.name == argument
                 result = discord.utils.find(check, bot.get_all_channels())
         else:
             channel_id = int(match.group(1))
@@ -185,13 +193,14 @@ class VoiceChannelConverter(IDConverter):
                 result = _get_from_guilds(bot, 'get_channel', channel_id)
 
         if not isinstance(result, discord.VoiceChannel):
-            raise BadArgument('Channel "{}" not found.'.format(self.argument))
+            raise BadArgument('Channel "{}" not found.'.format(argument))
 
         return result
 
 class ColourConverter(Converter):
-    def convert(self):
-        arg = self.argument.replace('0x', '').lower()
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        arg = argument.replace('0x', '').lower()
 
         if arg[0] == '#':
             arg = arg[1:]
@@ -205,47 +214,48 @@ class ColourConverter(Converter):
             return method()
 
 class RoleConverter(IDConverter):
-    def convert(self):
-        guild = self.ctx.message.guild
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        guild = ctx.message.guild
         if not guild:
             raise NoPrivateMessage()
 
-        match = self._get_id_match() or re.match(r'<@&([0-9]+)>$', self.argument)
-        params = dict(id=int(match.group(1))) if match else dict(name=self.argument)
+        match = self._get_id_match(argument) or re.match(r'<@&([0-9]+)>$', argument)
+        params = dict(id=int(match.group(1))) if match else dict(name=argument)
         result = discord.utils.get(guild.roles, **params)
         if result is None:
-            raise BadArgument('Role "{}" not found.'.format(self.argument))
+            raise BadArgument('Role "{}" not found.'.format(argument))
         return result
 
 class GameConverter(Converter):
-    def convert(self):
-        return discord.Game(name=self.argument)
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        return discord.Game(name=argument)
 
 class InviteConverter(Converter):
     @asyncio.coroutine
-    def convert(self):
+    def convert(self, ctx, argument):
         try:
-            invite = yield from self.ctx.bot.get_invite(self.argument)
+            invite = yield from ctx.bot.get_invite(argument)
             return invite
         except Exception as e:
             raise BadArgument('Invite is invalid or expired') from e
 
 class EmojiConverter(IDConverter):
     @asyncio.coroutine
-    def convert(self):
-        message = self.ctx.message
-        bot = self.ctx.bot
-
-        match = self._get_id_match() or re.match(r'<:[a-zA-Z0-9]+:([0-9]+)>$', self.argument)
+    def convert(self, ctx, argument):
+        match = self._get_id_match(argument) or re.match(r'<:[a-zA-Z0-9]+:([0-9]+)>$', argument)
         result = None
-        guild = message.guild
+        bot = ctx.bot
+        guild = ctx.guild
+
         if match is None:
             # Try to get the emoji by name. Try local guild first.
             if guild:
-                result = discord.utils.get(guild.emojis, name=self.argument)
+                result = discord.utils.get(guild.emojis, name=argument)
 
             if result is None:
-                result = discord.utils.get(bot.emojis, name=self.argument)
+                result = discord.utils.get(bot.emojis, name=argument)
         else:
             emoji_id = int(match.group(1))
 
@@ -257,7 +267,7 @@ class EmojiConverter(IDConverter):
                 result = discord.utils.get(bot.emojis, id=emoji_id)
 
         if result is None:
-            raise BadArgument('Emoji "{}" not found.'.format(self.argument))
+            raise BadArgument('Emoji "{}" not found.'.format(argument))
 
         return result
 
@@ -266,8 +276,9 @@ class clean_content(Converter):
         self.fix_channel_mentions = fix_channel_mentions
         self.use_nicknames = use_nicknames
 
-    def convert(self):
-        message = self.ctx.message
+    @asyncio.coroutine
+    def convert(self, ctx, argument):
+        message = ctx.message
         transformations = {}
 
         if self.fix_channel_mentions:
@@ -306,7 +317,7 @@ class clean_content(Converter):
             return transformations.get(obj.group(0), '')
 
         pattern = re.compile('|'.join(transformations.keys()))
-        result = pattern.sub(repl, self.argument)
+        result = pattern.sub(repl, argument)
 
         transformations = {
             '@everyone': '@\u200beveryone',
diff --git a/discord/ext/commands/core.py b/discord/ext/commands/core.py
index 48223895..0b05a1ae 100644
--- a/discord/ext/commands/core.py
+++ b/discord/ext/commands/core.py
@@ -202,13 +202,10 @@ class Command:
 
         if inspect.isclass(converter) and issubclass(converter, converters.Converter):
             instance = converter()
-            instance.prepare(ctx, argument)
-            ret = yield from discord.utils.maybe_coroutine(instance.convert)
+            ret = yield from instance.convert(ctx, argument)
             return ret
-
-        if isinstance(converter, converters.Converter):
-            converter.prepare(ctx, argument)
-            ret = yield from discord.utils.maybe_coroutine(converter.convert)
+        elif isinstance(converter, converters.Converter):
+            ret = yield from converter.convert(ctx, argument)
             return ret
 
         return converter(argument)
@@ -220,9 +217,6 @@ class Command:
                 converter = str if param.default is None else type(param.default)
             else:
                 converter = str
-        elif not inspect.isclass(type(converter)):
-            raise discord.ClientException('Function annotation must be a type')
-
         return converter
 
     @asyncio.coroutine