python - Splitting a list of dictionaries into several lists of dictionaries

Question

I've been whacking away at this for a while to no avail... Any help would be greatly appreciated.

I have:

[{'event': 0, 'voltage': 1, 'time': 0},
{'event': 0, 'voltage': 2, 'time': 1},
{'event': 1, 'voltage': 1, 'time': 2},
{'event': 1, 'voltage': 2, 'time': 3},
{'event': 2, 'voltage': 1, 'time': 4},
{'event': 2, 'voltage': 2, 'time': 5},
...]

and I want to split that list of dictionaries up per event like this (there can be arbitrarily many events):

list0 = [{'event': 0, 'voltage': 1, 'time': 0},
{'event': 0, 'voltage': 2, 'time': 1}]

list1 = [{'event': 1, 'voltage': 1, 'time': 2},
{'event': 1, 'voltage': 2, 'time': 3}]

list2 = [{'event': 2, 'voltage': 1, 'time': 4},
{'event': 2, 'voltage': 2, 'time': 5}]

listN = ...

Answer 1

use defaultdict

import collections

result = collections.defaultdict(list)

for d in dict_list:
    result[d['event']].append(d)

result_list = result.values()        # Python 2.x
result_list = list(result.values())  # Python 3

This way, you don't have to make any assumptions about how many different events there are or if there are any events missing.

This gives you a list of lists. If you want a dict indexed by event, I would probably use dict(d) if you plan on doing any random access.

As far as constructing a bunch of individual lists, I think that that's a bad idea. It will necessitate creating them as globals or using eval (or getting hacky in some other way) unless you know exactly how many there are going to be which you claim not to. It's best to just keep them in a container.