python - 在字典列表中添加元素

Question

我有一长串带有字符串索引和整数值的字典。许多键在字典中都是相同的，尽管不是全部。我想生成一个字典，其中键是单独字典中键的并集，值是每个字典中与该键对应的所有值的总和。 (例如，组合字典中键 'apple' 的值将是第一个中 'apple' 值的总和，加上第二个中 'apple' 值的总和，依此类推。)

我有以下内容，但它相当麻烦并且需要 ages 来执行。有没有更简单的方法来达到相同的结果？

comb_dict = {}  
for dictionary in list_dictionaries:  
    for key in dictionary:  
        comb_dict.setdefault(key, 0)  
        comb_dict[key] += dictionary[key]  
return comb_dict

Answer 1

这里有一些微基准测试表明 f2(见下文)可能是一种改进。 f2 使用 iteritems 可以避免在内部循环中进行额外的字典查找:

import collections
import string
import random

def random_dict():
    n=random.randint(1,26)
    keys=list(string.letters)
    random.shuffle(keys)
    keys=keys[:n]
    values=[random.randint(1,100) for _ in range(n)]    
    return dict(zip(keys,values))

list_dictionaries=[random_dict() for x in xrange(100)]

def f1(list_dictionaries):
    comb_dict = {}  
    for dictionary in list_dictionaries:  
        for key in dictionary:  
            comb_dict.setdefault(key, 0)  
            comb_dict[key] += dictionary[key]  
    return comb_dict

def f2(list_dictionaries):    
    comb_dict = collections.defaultdict(int)
    for dictionary in list_dictionaries:  
        for key,value in dictionary.iteritems():  
            comb_dict[key] += value
    return comb_dict

def union( dict_list ):
    all_keys = set()
    for d in dict_list:
        for k in d:
            all_keys.add( k )
    for key in all_keys:
        yield key, sum( d.get(key,0) for d in dict_list)

def f3(list_dictionaries):
    return dict(union( list_dictionaries ))

结果如下:

% python -mtimeit -s"import test" "test.f1(test.list_dictionaries)"
1000 loops, best of 3: 776 usec per loop
% python -mtimeit -s"import test" "test.f2(test.list_dictionaries)"
1000 loops, best of 3: 432 usec per loop    
% python -mtimeit -s"import test" "test.f3(test.list_dictionaries)"
100 loops, best of 3: 2.19 msec per loop