Throw exception when an argument could not be found.

AppendCommand.py

-import Command
+from Command import *
 from PrettyPrinter import pretty_print
 import TodoList
 from Utils import convert_todo_number, InvalidTodoNumberException
 
-class AppendCommand(Command.Command):
+class AppendCommand(Command):
     def __init__(self, p_args, p_todolist,
                  p_out=lambda a: None,
                  p_err=lambda a: None,
@@ -11,10 +11,8 @@ class AppendCommand(Command.Command):
         super(AppendCommand, self).__init__(p_args, p_todolist, p_out, p_err, p_prompt=lambda a: None)
 
     def execute(self):
-        number = self.argument(0)
-        if number:
         try:
-                number = convert_todo_number(number)
+            number = convert_todo_number(self.argument(0))
             text = " ".join(self.args[1:])
 
             if text:
@@ -23,7 +21,7 @@ class AppendCommand(Command.Command):
                 self.out(pretty_print(todo, [self.todolist.pp_number()]))
             else:
                 self.error(self.usage())
-            except InvalidTodoNumberException:
+        except (InvalidCommandArgument, InvalidTodoNumberException):
             self.error(self.usage())
         except TodoList.InvalidTodoException:
             self.error("Invalid todo number given.")

Command.py

+class InvalidCommandArgument(Exception):
+    pass
+
 class Command(object):
     def __init__(self, p_args, p_todolist,
                  p_out=lambda a: None,
@@ -37,13 +40,13 @@ class Command(object):
         """
         return False
 
-    def argument(self, p_number):
+    def argument(self, p_number, p_error=None):
         """ Retrieves a value from the argument list at the given position. """
         value = None
         try:
             value = self.args[p_number]
         except IndexError:
-            self.error(self.usage())
+            raise InvalidCommandArgument
 
         return value