Get the keys and the default dictionary of the given function's
        arguments

def _get_keys_defdict(self):
        '''Get the keys and the default dictionary of the given function's
        arguments
        '''
        # inspect argspecs
        argspec = inspect.getargspec(self.func)
        keys, defvals = argspec.args, argspec.defaults

        # convert to (list_of_argkeys, dict_of_default_keys)
        if defvals is None:
            return keys, None
        else:
            defvals = list(defvals)
            keys.reverse()
            defvals.reverse()
            defdict = dict(zip(keys, defvals))
            keys.reverse()
            return keys, defdict