词典列表中的值总和 [英] Sum of values in list of dictionaries

问题描述

我想获取列表中所有字典中每个键的总和值，如果其中一个字典中不存在键，则将其值视为0.

I want to get the sum values for each key in all dictionaries of a list, and if a key is not present in one of the dictionaries, then its value is considered 0.

假设我有两个这样的字典:

Suppose I have two dictionaries as such:

d1 = {'a' : 2, 'b' : 1, 'c' : 1}
d2 = {'a' : 3, 'b' : 1.1, 'd' : 2}
mylist = [d1, d2]

，我想定义一个 sum 函数，使

and I would like to define a sum function such that

>>> sum(mylist)
{'a' : 5, 'b' : 2.1, 'c' : 1, 'd' : 2}

如果我只有两个字典，我可以做

If I only have two dictionaries, I can do

>>> for key, value in d2.items():
...    try:
...        d1[key] -= value
...    except KeyError: #if the key isn't in d1
...        d1[key] = -value

>>> d1
{'a' : 5, 'b' : 2.1, 'c' : 1, 'd' : 2}

但这不能扩展到任意数量的词典.

But this is not extendable to an arbitrary number of dictionaries.

我也尝试过

>>> {k: sum(e[k] for e in mylist) for k in mylist[0]}
{'a' : 5, 'b' : 2.1, 'c' : 1}

但是，这并没有给我没有在第一个列表中的元素的总和(在我的示例中，我缺少'd'的总和).

But this doesn't give me the sum for elements that aren't in the first list (I'm missing the sum for 'd' in my example).

我可以使用所有可能的键创建字典并将其添加到列表的开头

I could create a dictionary with all of the possible keys and add it to the front of my list

>>> d0 = {'a' : 0, 'b' : 0, 'c' : 0, 'd' : 0}
>>> newlist = [d0, d1, d2]
>>> {k: sum(e[k] for e in newlist) for k in newlist[0]}
{'a' : 5, 'b' : 2.1, 'c' : 1, 'd' : 2}

但是创建 d0 会很乏味.

我还可以使用 collections

>>> counterlist = [Counter(d) for d in mylist]
>>> result = Counter()
>>> for c in counterlist:
...    result.update(c)
>>> dict(result)

但是我对来回切换到 Counter 不太满意.

But I'm not too happy about switching back and forth to Counter.

或者，我可以实现类似更新"的功能

Or, I could implement an 'update-like' function

>>> def add(e, f):
...    for key, value in f.items():
...        try:
...            e[key] -= value
...        except KeyError:
...            e[key] = -value

>>> result = dict()
>>> for d in mylist:
...    add(result, d)
>>> result
{'a' : 5, 'b' : 2.1, 'c' : 1, 'd' : 2}

但是，这让我感到自己正在重新发明轮子.

But this makes me feel like I'm reinventing the wheel.

有没有更多的Python方式来做到这一点?

Is there a more pythonic way of doing this?

推荐答案

首先从您的词典列表中获取所有键并设置新词典:

First get all keys and set up a new dictionary from your list of dictionaries:

d1 = {'a' : 2, 'b' : 1, 'c' : 1}
d2 = {'a' : 3, 'b' : 1.1, 'd' : 2}
mylist = [d1, d2]
sum_dict = dict.fromkeys(set().union(*mylist), 0)

在那之后，简单地遍历字典和键列表很简单:

After that that is simple to just iterate over the list of dictionaries and the keys:

for d in mylist:
    for k in d.keys():
        sum_dict[k] += d[k]

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