python 合并字典，相同 key 的 value 如何相加？

Question

x = { 'apple': 1, 'banana': 2 }
y = { 'banana': 10, 'pear': 11 }

需要把两个字典合并，最后输出结果是：

{ 'apple': 1, 'banana': 12, 'pear': 11 }

Answer 1

利用collections.Counter可轻松办到

>>> x = { 'apple': 1, 'banana': 2 }
>>> y = { 'banana': 10, 'pear': 11 }
>>> from collections import Counter
>>> X,Y = Counter(x), Counter(y)
>>> z = dict(X+Y)
>>> z

本人的写法：

>>>from collections import Counter
>>>dict(Counter(x)+Counter(y))

Answer 2

上面两位的答案都是对的，我做了一下性能对比，你可以看下。

from collections import Counter
x = { 'apple': 1, 'banana': 2 }
y = { 'banana': 10, 'pear': 11 }
def f():
    i = 0
    while 1:
        if i == 10000:
            break
        for k, v in y.items():
            if k in x.keys():
                x[k] += v
            else:
                x[k] = v
        i += 1


def g():
    i = 0
    while 1:
        if i == 10000:
            break
        X,Y = Counter(x), Counter(y)
        z = dict(X+Y)
        i += 1

if __name__ == "__main__":
    import timeit
    tf = timeit.timeit(f, "from __main__ import f", number=10)
    tg = timeit.timeit(g, "from __main__ import g", number=10)
    print "f: %s\ng: %s" % (tf, tg)

输出：

f: 0.138742800623
g: 1.37559758279

我的建议是，简单的功能自己手写就好。

Answer 3

最后的结果中 banana 是 12 而不是 2 吧

>>> x = { 'apple': 1, 'banana': 2 }
>>> y = { 'banana': 10, 'pear': 11 }
>>> for k, v in y.items():
...     if k in x.keys():
...         x[k] += v
...     else:
...         x[k] = v
... 
>>> x
{'pear': 11, 'apple': 1, 'banana': 12}
>>>