<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def gen_enum_completions(self, arg_name):
 """ generates dynamic enumeration completions """

        try:  # if enum completion
            for choice in self.cmdtab[self.current_command].arguments[arg_name].choices:
                if self.validate_completion(choice):
                    yield Completion(choice, -len(self.unfinished_word))

        except TypeError:  # there is no choices option
            pass