Pythonが同じキーを持つ辞書の加算減算

In [1]: from collections import Counter

In [2]: x = { 'apple': 1, 'banana': 2 } 

In [3]: y = { 'banana': 10, 'pear': 11 }

In [4]: dict(Counter(x)+Counter(y)) 
Out[4]: {'apple': 1, 'banana': 12, 'pear': 11}

In [5]: dict(Counter(y)-Counter(x)) 
Out[5]: {'banana': 8, 'pear': 11}