Combine multiple dictionaries with the same schema

Question

Say I have a list of dictionaries of unknown length and unknown - but consistent - schema, as follows. Note that the values are always lists of the same length.

res = [{'x': [2, 1], 'v': [49280, 536704]}, {'x': [1, 4], 'v': [12336, 340000]}, {'x': [5, 6], 'v': [524360, 151624]}, {'x': [7, 1], 'v': [94280, 81968]}, {'x': [1, 1], 'v': [241856, 335904]}, {'x': [7, 7], 'v': [463016, 598040]}, {'x': [2, 9], 'v': [606256, 422016]}, {'x': [1, 1], 'v': [151680, 1237120]}]

Is there an efficient way to merge combine these together into a single dictionary? The final result for this example would be:

 {'x': [2, 1, 1, 4, 5, 6, 7, 1, 1, 1, 7, 7, 2, 9, 1, 1], 'v': [49280, 536704, 12336, 340000, 524360, 151624, 94280, 81968, 241856, 335904, 463016, 598040, 606256, 422016, 151680, 1237120]}

Working solution

This works, but I wonder if a one-liner for this operation?

output_dct = {} for dct in res: for k, v in dct.items(): output_dct[k] = v if k not in output_dct.keys() else output_dct[k] + v

Rakesh · Accepted Answer · 2020-05-07 14:16:31Z

A slightly better approach using dict.setdefault

Ex:

output_dct = {} for dct in res: for k, v in dct.items(): output_dct.setdefault(k, []).extend(v)

or using collections.defaultdict

from collections import defaultdict output_dct = defaultdict(list) for dct in res: for k, v in dct.items(): output_dct[k].extend(v) print(output_dct)

Output:

{'x': [2, 1, 1, 4, 5, 6, 7, 1, 1, 1, 7, 7, 2, 9, 1, 1], 'v': [49280, 536704, 12336, 340000, 524360, 151624, 94280, 81968, 241856, 335904, 463016, 598040, 606256, 422016, 151680, 1237120]}

Samwise · Accepted Answer · 2020-05-07 14:18:03Z

A one-liner, you say? To the comprehension-mobile!

>>> {k: sum((d[k] for d in res), []) for d in res for k in d} {'x': [2, 1, 1, 4, 5, 6, 7, 1, 1, 1, 7, 7, 2, 9, 1, 1], 'v': [49280, 536704, 12336, 340000, 524360, 151624, 94280, 81968, 241856, 335904, 463016, 598040, 606256, 422016, 151680, 1237120]}

(edit: practically speaking, I'd probably do this with a defaultdict like Rakesh did, because nested comprehensions are compact on the page but aren't necessarily obvious to other readers of the code.)

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