[commands] Add Context.reinvoke and Command.root_parent

Context.reinvoke would be the new way to bypass checks and cooldowns.
However, with its addition comes a change in the invocation order of
checks, callbacks, and cooldowns. While previously cooldowns would
trigger after command argument parsing, the new behaviour parses
cooldowns before command argument parsing.

The implication of this change is that Context.args and Context.kwargs
will no longer be filled properly.

discord/ext/commands/core.py

@@ -282,6 +282,24 @@ class Command:
 
         return ' '.join(reversed(entries))
 
+    @property
+    def root_parent(self):
+        """Retrieves the root parent of this command.
+
+        If the command has no parents then it returns ``None``.
+
+        For example in commands ``?a b c test``, the root parent is
+        ``a``.
+        """
+        entries = []
+        command = self
+        while command.parent is not None:
+            command = command.parent
+            entries.append(command)
+        entries.append(None)
+        entries.reverse()
+        return entries[-1]
+
     @property
     def qualified_name(self):
         """Retrieves the fully qualified command name.
@@ -350,7 +368,6 @@ class Command:
             if not view.eof:
                 raise TooManyArguments('Too many arguments passed to ' + self.qualified_name)
 
-
     @asyncio.coroutine
     def _verify_checks(self, ctx):
         if not self.enabled:
@@ -407,7 +424,6 @@ class Command:
     def prepare(self, ctx):
         ctx.command = self
         yield from self._verify_checks(ctx)
-        yield from self._parse_arguments(ctx)
 
         if self._buckets.valid:
             bucket = self._buckets.get_bucket(ctx)
@@ -415,6 +431,7 @@ class Command:
             if retry_after:
                 raise CommandOnCooldown(bucket, retry_after)
 
+        yield from self._parse_arguments(ctx)
         yield from self.call_before_hooks(ctx)
 
     def reset_cooldown(self, ctx):
@@ -440,6 +457,24 @@ class Command:
         injected = hooked_wrapped_callback(self, ctx, self.callback)
         yield from injected(*ctx.args, **ctx.kwargs)
 
+    @asyncio.coroutine
+    def reinvoke(self, ctx, *, call_hooks=False):
+        ctx.command = self
+        yield from self._parse_arguments(ctx)
+
+        if call_hooks:
+            yield from self.call_before_hooks(ctx)
+
+        ctx.invoked_subcommand = None
+        try:
+            yield from self.callback(*ctx.args, **ctx.kwargs)
+        except:
+            ctx.command_failed = True
+            raise
+        finally:
+            if call_hooks:
+                yield from self.call_after_hooks(ctx)
+
     def error(self, coro):
         """A decorator that registers a coroutine as a local error handler.
 
@@ -821,6 +856,44 @@ class Group(GroupMixin, Command):
             view.previous = previous
             yield from super().invoke(ctx)
 
+    @asyncio.coroutine
+    def reinvoke(self, ctx, *, call_hooks=False):
+        early_invoke = not self.invoke_without_command
+        if early_invoke:
+            ctx.command = self
+            yield from self._parse_arguments(ctx)
+
+            if call_hooks:
+                yield from self.call_before_hooks(ctx)
+
+        view = ctx.view
+        previous = view.index
+        view.skip_ws()
+        trigger = view.get_word()
+
+        if trigger:
+            ctx.subcommand_passed = trigger
+            ctx.invoked_subcommand = self.all_commands.get(trigger, None)
+
+        if early_invoke:
+            try:
+                yield from self.callback(*ctx.args, **ctx.kwargs)
+            except:
+                ctx.command_failed = True
+                raise
+            finally:
+                if call_hooks:
+                    yield from self.call_after_hooks(ctx)
+
+        if trigger and ctx.invoked_subcommand:
+            ctx.invoked_with = trigger
+            yield from ctx.invoked_subcommand.reinvoke(ctx, call_hooks=call_hooks)
+        elif not early_invoke:
+            # undo the trigger parsing
+            view.index = previous
+            view.previous = previous
+            yield from super().reinvoke(ctx, call_hooks=call_hooks)
+
 # Decorators
 
 def command(name=None, cls=None, **attrs):