<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def lists(self, value, key=None):
 """ Get a list with the values of a given key :rtype: list """

        results = map(lambda x: x[value], self._items)

        return list(results)