<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def copy(self, key):
 """Copy the list to a new list. WARNING: If key exists, it clears it before copying. """

        copy = List(key, self.db)
        copy.clear()
        copy.extend(self)
        return copy