[commands] Add support for cogs.

Cogs are basically class instances that have commands and event
listeners. They allow for better organisation and grouping of
commands and state. Similar to subclassing discord.Client.

discord/ext/commands/core.py

@@ -71,6 +71,9 @@ class Command:
         If the command is invoked while it is disabled, then
         :exc:`DisabledCommand` is raised to the :func:`on_command_error`
         event. Defaults to ``True``.
+    parent : Optional[command]
+        The parent command that this command belongs to. ``None`` is there
+        isn't one.
     checks
         A list of predicates that verifies if the command could be executed
         with the given :class:`Context` as the sole parameter. If an exception
@@ -90,6 +93,9 @@ class Command:
         signature = inspect.signature(callback)
         self.params = signature.parameters.copy()
         self.checks = kwargs.get('checks', [])
+        self.module = inspect.getmodule(callback)
+        self.instance = None
+        self.parent = None
 
     def _receive_item(self, message, argument, regex, receiver, generator):
         match = re.match(regex, argument)
@@ -181,14 +187,25 @@ class Command:
 
     def _parse_arguments(self, ctx):
         try:
-            ctx.args = []
+            ctx.args = [] if self.instance is None else [self.instance]
             ctx.kwargs = {}
             args = ctx.args
             kwargs = ctx.kwargs
 
             first = True
             view = ctx.view
-            for name, param in self.params.items():
+            iterator = iter(self.params.items())
+
+            if self.instance is not None:
+                # we have 'self' as the first parameter so just advance
+                # the iterator and resume parsing
+                try:
+                    next(iterator)
+                except StopIteration:
+                    fmt = 'Callback for {0.name} command is missing "self" parameter.'
+                    raise discord.ClientException(fmt.format(self))
+
+            for name, param in iterator:
                 if first and self.pass_context:
                     args.append(ctx)
                     first = False
@@ -272,6 +289,9 @@ class GroupMixin:
         if not isinstance(command, Command):
             raise TypeError('The command passed must be a subclass of Command')
 
+        if isinstance(self, Command):
+            command.parent = self
+
         if command.name in self.commands:
             raise discord.ClientException('Command {0.name} is already registered.'.format(command))