list - Python - sum values in dictionary

75
votes

I have got pretty simple list:

example_list = [
    {'points': 400, 'gold': 2480},
    {'points': 100, 'gold': 610},
    {'points': 100, 'gold': 620},
    {'points': 100, 'gold': 620}
]

How can I sum all gold values? I'm looking for nice oneliner.

Now I'm using this code (but it's not the best solution):

total_gold = 0
for item in example_list:
    total_gold += example_list["gold"]
3
pythonlistdictionary
You shouldn't use list as the name of a local variable - doing so shadows the built-in list type and can cause problems. - g.d.d.c
You're right, I'm use this only for this example. - Mateusz Jagiełło
@sigo -- In this case, a nice one-liner exists, but generally, restricting answers to "nice one-liners" is probably a bad idea (as nicer multi-liners might exist). - mgilson

3 Answers

175
votes
sum(item['gold'] for item in myList)
21
votes

If you're memory conscious:

sum(item['gold'] for item in example_list)

If you're extremely time conscious:

sum([item['gold'] for item in example_list])

In most cases just use the generator expression, as the performance increase is only noticeable on a very large dataset/very hot code path.

See this answer for an explanation of why you should avoid using map.

See this answer for some real-world timing comparisons of list comprehension vs generator expressions.

8
votes

If you prefer map, this works too:

 import operator
 total_gold = sum(map(operator.itemgetter('gold'),example_list))

But I think the generator posted by g.d.d.c is significantly better. This answer is really just to point out the existence of operator.itemgetter.