如何在列表中使用相同的键对词典值进行求和？

Question

I have this list:

list1 = [
    {'currency': 'USD', 'value': 10},
    {'currency': 'USD', 'value': 12},
    {'currency': 'EUR', 'value': 11},
    {'currency': 'EUR', 'value': 15},
    {'currency': 'EUR', 'value': 17},
    {'currency': 'GBP', 'value': 13},
]

How do I combine the dictionaries so I get this list from list1?

list2 = [
    {'currency': 'USD', 'value': 22},
    {'currency': 'EUR', 'value': 43},
    {'currency': 'GBP', 'value': 13},
]

Answer 1

Use a dictionary:

d = {}
for x in list1:
     c, v = x["currency"], x["value"]
     d[c] = d.get(c, 0) + v
# {'EUR': 43, 'GBP': 13, 'USD': 22}

Then either use that dict directly (I would recommend that) or turn it back into your list-of-dicts format, use a list-comprehension:

>>> [{"currency": k, "value": v} for k, v in d.items()]
[{'currency': 'USD', 'value': 22},
 {'currency': 'EUR', 'value': 43},
 {'currency': 'GBP', 'value': 13}]

Answer 2

Using collections.defaultdict .

Demo:

import collections
d = collections.defaultdict(int)
list1 = [
    {'currency': 'USD', 'value': 10},
    {'currency': 'USD', 'value': 12},
    {'currency': 'EUR', 'value': 11},
    {'currency': 'EUR', 'value': 15},
    {'currency': 'EUR', 'value': 17},
    {'currency': 'GBP', 'value': 13},
]
for i in list1:
    d[i['currency']] += i["value"]

print( [{'currency': k, 'value': v} for k,v in d.items()] )

Output:

[{'currency': 'USD', 'value': 22}, {'currency': 'GBP', 'value': 13}, {'currency': 'EUR', 'value': 43}]

Answer 3

you can use Counter to calculate the summs

from collections import Counter

c = Counter()
for d in list1:
    cur, v = d.get('currency'), d.get('value')
    c.update({cur: v})

print(c)
Counter({'EUR': 43, 'USD': 22, 'GBP': 13})

and after generate the output:

list2 = [{'currency': cur, 'value': v} for cur, v in c.items()]
print(list2)
[{'currency': 'USD', 'value': 22}, {'currency': 'GBP', 'value': 13}, {'currency': 'EUR', 'value': 43}]

Answer 4

This should do it:

list1 = [
    {'currency': 'USD', 'value': 10},
    {'currency': 'USD', 'value': 12},
    {'currency': 'EUR', 'value': 11},
    {'currency': 'EUR', 'value': 15},
    {'currency': 'EUR', 'value': 17},
    {'currency': 'GBP', 'value': 13},
]

valuePairs = {}

for d in list1:
    curr = d['currency']
    val = d['value']

    if curr in valuePairs:
        valuePairs[curr] += val
    else:
        valuePairs[curr] = val

solution = [{'currency': k, 'value': v} for k, v in valuePairs.items()]

Answer 5

you could also do:

import itertools as it
list1 = [
    {'currency': 'USD', 'value': 10},
    {'currency': 'USD', 'value': 12},
    {'currency': 'EUR', 'value': 11},
    {'currency': 'EUR', 'value': 15},
    {'currency': 'EUR', 'value': 17},
    {'currency': 'GBP', 'value': 13},
]

kfunc = lambda x: x['currency']

groups = it.groupby(sorted(list1, key=kfunc), kfunc)
result = [{'currency':k, 'value':sum(x['value'] for x in g)} for k, g in groups]
print(result)

Answer 6

You can also one-line it:

[{"currency": k, "value": sum([d["value"] for d in list1 if d["currency"] == k])} for k in list(set([d["currency"] for d in list1]))]

list(set([d["currency"] for d in list1])) gives you the unique list of currencies

You sum then values of each unique currency to make the new dict.