[commands] Add CommandNotFound error.

discord/ext/commands/bot.py

@@ -31,6 +31,7 @@ import inspect
 from .core import GroupMixin
 from .view import StringView
 from .context import Context
+from .errors import CommandNotFound
 
 class Bot(GroupMixin, discord.Client):
     """Represents a discord bot.
@@ -203,18 +204,21 @@ class Bot(GroupMixin, discord.Client):
 
         view.skip_ws()
         invoker = view.get_word()
-        if invoker in self.commands:
-            command = self.commands[invoker]
         tmp = {
             'bot': self,
             'invoked_with': invoker,
             'message': message,
             'view': view,
-                'command': command
         }
         ctx = Context(**tmp)
         del tmp
+        if invoker in self.commands:
+            command = self.commands[invoker]
+            ctx.command = command
             yield from command.invoke(ctx)
+        else:
+            exc = CommandNotFound('Command "{}" is not found'.format(invoker))
+            self.dispatch('command_error', exc, ctx)
 
     @asyncio.coroutine
     def on_message(self, message):

discord/ext/commands/core.py

@@ -334,7 +334,6 @@ class Group(GroupMixin, Command):
         if ctx.invoked_subcommand:
             yield from ctx.invoked_subcommand.invoke(ctx)
 
-
 # Decorators
 
 def command(name=None, cls=None, **attrs):

discord/ext/commands/errors.py

@@ -27,7 +27,7 @@ from discord.errors import DiscordException
 
 
 __all__ = [ 'CommandError', 'MissingRequiredArgument', 'BadArgument',
-           'NoPrivateMessage', 'CheckFailure' ]
+           'NoPrivateMessage', 'CheckFailure', 'CommandNotFound' ]
 
 class CommandError(DiscordException):
     """The base exception type for all command related errors.
@@ -40,6 +40,14 @@ class CommandError(DiscordException):
     """
     pass
 
+class CommandNotFound(CommandError):
+    """Exception raised when a command is attempted to be invoked
+    but no command under that name is found.
+
+    This is not raised for invalid subcommands, rather just the
+    initial main command that is attempted to be invoked.
+    """
+
 class MissingRequiredArgument(CommandError):
     """Exception raised when parsing a command and a parameter
     that is required is not encountered.