Best way to combine a list of dicts into one dict in python?

Question

I've been working with a long list of dicts for a while and can't seem to wrap my head around how to sort it the way I'd like. Here's a shorter example:

artist_movement = [
{'movement': 'Abstract Expressionism', 'artist': 'William Baziotes'}, 
{'movement': 'Modern Art', 'artist': 'Alexander Calder'}, 
{'movement': 'Abstract Expressionism', 'artist': 'Grace Hartigan'},
{'movement': 'Cubism', 'artist': 'Pablo Picasso'}, 
{'movement': 'Cubism', 'artist': 'Peter Blume'}, 
{'movement': 'Abstract Expressionism', 'artist': 'Norman Wilfred Lewis'},
{'movement': 'Modern Art', 'artist': 'Lucian Freud'}
]

I'd like to sort the artists by movement in in a similar way to this:

artist_by_movement = [
{'Abstract Expressionism':['William Baziotes', 'Grace Hartigan', 'Norman Wilfred Lewis']},
{'Modern Art':['Alexander Calder', 'Lucian Freud']},
{'Cubism':['Peter Blume', 'Pablo Picasso']}
]

Thanks for any help!

Have you looked on SO/google for something similar? Merging dicts comes up a lot... — Andras Deak -- Слава Україні, Dec 01 '16 at 23:24
Does [this](http://stackoverflow.com/questions/2365921/merging-python-dictionaries) help? — Andras Deak -- Слава Україні, Dec 01 '16 at 23:24
Yes, thanks, I looked for a few hours for a good explanation that related directly to my problem (merging a list of dicts by key) but didn't find anything. — djangonoob, Dec 01 '16 at 23:25
Actually, that link may help. I'll have another go at it. Thanks! — djangonoob, Dec 01 '16 at 23:27
Just a note that your artists_by_movement dictionary doesn't work as written. You have multiple values for the same key, which overwrite each other. — Reid, Dec 01 '16 at 23:36
@djangonoob try something like this: artists_by_movement2 = [{'Abstract Expressionism':['William Baziotes', 'Grace Hartigan', 'Norman Wilfred Lewis']}, {'Modern Art':['Alexander Calder', 'Lucian Freud']}, {'Cubism':['Peter Blume', 'Pablo Picasso']}] ##Because the movement & artist relationship is always there, it is implied by the structure and you don't need to explicitly use "movement" as a key. — Reid, Dec 01 '16 at 23:55
@Reid thanks. that'll give me something to aim for as I keep hammering away at it. Still learning! I edited my question to reflect your advised format. — djangonoob, Dec 02 '16 at 00:04

score 4 · Accepted Answer · answered Dec 01 '16 at 23:59

You can use defaultdict to create a dict whose keys are the movements and values are a list of artists in that movement. All defaultdict does is auto-create a list when a new key is seen.

import collections

artist_movement = [
{'movement': 'Abstract Expressionism', 'artist': 'William Baziotes'}, 
{'movement': 'Modern Art', 'artist': 'Alexander Calder'}, 
{'movement': 'Abstract Expressionism', 'artist': 'Grace Hartigan'},
{'movement': 'Cubism', 'artist': 'Pablo Picasso'}, 
{'movement': 'Cubism', 'artist': 'Peter Blume'}, 
{'movement': 'Abstract Expressionism', 'artist': 'Norman Wilfred Lewis'},
{'movement': 'Modern Art', 'artist': 'Lucian Freud'}
]

artist_by_movement = collections.defaultdict(list)
for d in artist_movement:
    artist_by_movement[d['movement']].append(d['artist'])

If you want a more traditional index to the original dictionaries (perhaps there is more interesting information in them) you could do

artist_by_movement = collections.defaultdict(list)
for d in artist_movement:
    artist_by_movement[d['movement']].append(d)

Works like a charm! Thanks so much. – djangonoob Dec 02 '16 at 00:10 — djangonoob, Dec 02 '16 at 00:10

Best way to combine a list of dicts into one dict in python?

1 Answers1