How do I convert a list of dictionaries to a dictionary of lists in Python?

Question

It may be a classical question in Python, but I haven't found the answer yet.

I have a list of dictionaries, these dictionaries have similar keys. It looks like this:

 [{0: myech.MatchingResponse at 0x10d6f7fd0, 
   3: myech.MatchingResponse at 0x10d9886d0,
   6: myech.MatchingResponse at 0x10d6f7d90,
   9: myech.MatchingResponse at 0x10d988ad0},
  {0: myech.MatchingResponse at 0x10d6f7b10,
   3: myech.MatchingResponse at 0x10d6f7f90>}]

I would like to get a new dictionary with [0,3,6,9] as keys, and lists of " myech.MatchingResponse" as values.

Of course I can do this using a simple loop but I was wondering if there is a more efficient solution.

For the reverse (from dict-of-lists to list-of-dicts) see: https://stackoverflow.com/q/1780174/1959808 — 0 _, Sep 20 '17 at 01:45

score 19 · Accepted Answer · answered Jul 12 '12 at 11:17

19

import collections

result = collections.defaultdict(list)

for d in dictionaries:
    for k, v in d.items():
        result[k].append(v)

answered Jul 12 '12 at 11:17

Otto Allmendinger

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1

This is very slow! – salomonvh Oct 14 '16 at 09:32

score 2 · Answer 2 · edited Jul 12 '12 at 11:33

2

let's say your list is assigned to a variable called mylist.

mydic = {}
for dic in mylist:
    for key, value in dic.items():
        if key in mydic:
            mydic[key].append(value)
        else:
            mydic[key] = [value]

edited Jul 12 '12 at 11:33

jamylak

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answered Jul 12 '12 at 11:20

bigblind

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4

use `dict.setdefault` or `collections.defaultdict` instead of this! :D – jamylak Jul 12 '12 at 11:22
Also this won't work since iterating through a dictionary iterates through it's keys so this, `for key, value in dic`, will raise an error. change to `for key, value in dic.items()`. Edit: I just changed it for you – jamylak Jul 12 '12 at 11:30
Why is is so wrong to initialize a dictionary with dic = {} ? – lizzie Jul 12 '12 at 12:08
1

@lizzie I don't see anything wrong with that. Just don't call it dict or you are shadowing the builtin dict class and you won't be able to access it since your variable has taken the name. – jamylak Jul 12 '12 at 23:18
that sounds weird, a variable shadowing it's own type :p – bigblind Jul 13 '12 at 04:19

Maria Zverina · Answer 3 · 2012-07-12T13:37:48.893

2

It's possible to do this with dict comprehension as well ... could be one line, but I've kept it as two lines for clarity. :)

from itertools import chain

all_keys = set(chain(*[x.keys() for x in dd]))
print {k : [d[k] for d in dd if k in d] for k in all_keys}

Results in:

{0: ['a', 'x'], 9: ['d'], 3: ['b', 'y'], 6: ['c']}

edited Jul 12 '12 at 13:37

answered Jul 12 '12 at 11:51

Maria Zverina

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this is not what the asker is after – Otto Allmendinger Jul 12 '12 at 12:42
@OttoAllmendinger Ah .. I misunderstood the problem ... please have a look at the corrected solution :) – Maria Zverina Jul 12 '12 at 13:38
I'm not sure about the performance but now it is a valid answer – Otto Allmendinger Jul 12 '12 at 13:46

score 0 · Answer 4 · answered Sep 04 '19 at 07:08

0

If you have a list of dictionaries with identical keys in each, you can convert these to a dictionary of lists as in the following example, (which some would consider more pythonic than some of the other answers).

d = []
d.append({'a':1,'b':2})
d.append({'a':4,'b':3}) 
print(d)                                                               
[{'a': 1, 'b': 2}, {'a': 4, 'b': 3}]

newdict = {}
for k,v in d[0].items():
    newdict[k] = [x[k] for x in d]

print(newdict)
{'a': [1, 4], 'b': [2, 3]}

answered Sep 04 '19 at 07:08

vschmidt

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`If you have a lists of dictionaries with identical keys`. Which is explicitly ***not*** the case in the question that was actually asked. So, this is an answer to a different question? I propose a shorter answer; `42`, which is *also* an answer to a different question. – MatBailie Nov 20 '21 at 13:02

How do I convert a list of dictionaries to a dictionary of lists in Python?

4 Answers4