CODEWITHSUNDEEP
pythonpython-3.xdictionarymerge
ruancomelli
ruancomelli

Reputation: 730

How to unpack or merge a variadic list of dictionaries in Python?

According to this answer, in Python 3.5 or greater, it is possible to merge two dictionaries x and y by unpacking them:

z = {**x, **y}

Is it possible to unpack a variadic list of dictionaries?

Something like

def merge(*dicts):
    return {***dicts} # this fails, of course. What should I use here?

For instance, I would expect that

list_of_dicts = [{'a': 1, 'b': 2}, {'c': 3}, {'d': 4}]
{***list_of_dicts} == {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Note that this question is not about how to merge lists of dictionaries since the link above provides an answer to this. The question here is: is it possible, and how, to unpack lists of dictionaries?

As stated in the comments, this question is very similar to this one. However, unpacking a list of dictionaries is different from simply merging them. Supposing that there was an operator *** designed to unpack lists of dictionaries, and given

def print_values(a, b, c, d):
    print('a =', a)
    print('b =', b)
    print('c =', c)
    print('d =', d)

list_of_dicts = [{'a': 1, 'b': 2}, {'c': 3}, {'d': 4}]

it would be possible to write

print_values(***list_of_dicts)

instead of

print_values(**merge(list_of_dicts))

Upvotes: 12

Views: 10734

Answers (8)

Subham
Subham

Reputation: 411

When you *dicts its put in as a tuple, you can pull the list out with d[0], then use this comprehension for nonuniform keys

list_of_dicts = [{'a': 1, 'b': 2}, {'c': 3}, {'d': 4}]

def merge(*dicts):    
    return dict( j for i in dicts[0] for j in i.items())

print(merge(list_of_dicts)) 

{'a': 1, 'b': 2, 'c': 3, 'd': 4}
[Program finished]

Upvotes: 0

Prem Anand
Prem Anand

Reputation: 2537

You can use reduce to merge two dicts at a time using dict.update

>>> from functools import reduce
>>> lst = [{'a':1}, {'b':2}, {'c':1}, {'d':2}]
>>> reduce(lambda d1, d2: d1.update(d2) or d1, lst, {})
{'a': 1, 'b': 2, 'c': 1, 'd': 2}

Upvotes: 0

Prem Anand
Prem Anand

Reputation: 2537

You can just use a list comprehension to iterate over all the dicts in the list and then iterate over each if those dicts' items and finally convert them to dict

>>> lst = [{'a':1}, {'b':2}, {'c':1}, {'d':2}]
>>> dict(kv for d in lst for kv in d.items())
{'a': 1, 'b': 2, 'c': 1, 'd': 2}

Upvotes: 0

chepner
chepner

Reputation: 531085

There's no syntax for that, but you can use itertools.chain to concatenate the key/value tuples from each dict into a single stream that dict can consume.

from itertools import chain


def merge(*dicts):
    return dict(chain.from_iterable(d.items() for d in dicts))

You can also unpack a list created by a list comprehension as well:

def merge(*dicts):
    return dict(*[d.items() for d in dicts])

Upvotes: 6

Andy L.
Andy L.

Reputation: 25239

Another solution is using collections.ChainMap

from collections import ChainMap

dict(ChainMap(*list_of_dicts[::-1]))

Out[88]: {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Upvotes: 7

Mykola Zotko
Mykola Zotko

Reputation: 17814

To merge multiple dictionaries you can use the function reduce:

from functools import reduce

lst = [{'a': 1, 'b': 2}, {'c': 3}, {'d': 4}]

reduce(lambda x, y: dict(**x, **y), lst)
# {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Upvotes: 2

Jevs
Jevs

Reputation: 25

You could use list comprehension and put this iterable object as an argument to dict

def merge(*dicts):
    lst = [*[d.items() for d in dicts]]
    return dict(lst)

Upvotes: 1

Jan
Jan

Reputation: 43169

You could just iterate over the list and use update:

lst = [{'a': 1, 'b': 2}, {'c': 3}, {'d': 4}]

dct = {}
for item in lst:
    dct.update(item)

print(dct)
# {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Upvotes: 5

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