<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def destroy(self):
        '''Unsubscribes callback from observable'''
        self._observable.release(self._key, self._callback)
        self._observable = None
        self._key = None
        self._callback = None