How to sum dict elements

Question

In Python, I have list of dicts:

dict1 = [{'a':2, 'b':3},{'a':3, 'b':4}]

I want one final dict that will contain the sum of all dicts. I.e the result will be: {'a':5, 'b':7}

N.B: every dict in the list will contain same number of key, value pairs.

Answer 1

A little ugly, but a one-liner:

dictf = reduce(lambda x, y: dict((k, v + y[k]) for k, v in x.iteritems()), dict1)

Answer 2

The following code shows one way to do it:

dict1 = [{'a':2, 'b':3},{'a':3, 'b':4}]

final = {}
for k in dict1[0].keys():           # Init all elements to zero.
    final[k] = 0
for d in dict1:
    for k in d.keys():
        final[k] = final[k] + d[k]  # Update the element.

print final

This outputs:

{'a': 5, 'b': 7}

as you desired.

Or, as inspired by kriss, better but still readable:

dict1 = [{'a':2, 'b':3},{'a':3, 'b':4}]

final = {}
for d in dict1:
    for k in d.keys():
        final[k] = final.get(k,0) + d[k]

print final

I pine for the days of the original, readable Python :-)

Answer 3

This might help:

def sum_dict(d1, d2):
    for key, value in d1.items():
        d1[key] = value + d2.get(key, 0)
    return d1

>>> dict1 = [{'a':2, 'b':3},{'a':3, 'b':4}]
>>> reduce(sum_dict, dict1)
{'a': 5, 'b': 7}

Answer 4

Leveraging sum() should get better performance when adding more than a few dicts

>>> dict1 = [{'a':2, 'b':3},{'a':3, 'b':4}]
>>> from operator import itemgetter
>>> {k:sum(map(itemgetter(k), dict1)) for k in dict1[0]}        # Python2.7+
{'a': 5, 'b': 7}
>>> dict((k,sum(map(itemgetter(k), dict1))) for k in dict1[0])  # Python2.6
{'a': 5, 'b': 7}

adding Stephan's suggestion

>>> {k: sum(d[k] for d in dict1) for k in dict1[0]}            # Python2.7+
{'a': 5, 'b': 7}
>>> dict((k, sum(d[k] for d in dict1)) for k in dict1[0])      # Python2.6
{'a': 5, 'b': 7}

I think Stephan's version of the Python2.7 code reads really nicely

Answer 5

You can use the collections.Counter

counter = collections.Counter()
for d in dict1: 
    counter.update(d)

Or, if you prefer oneliners:

functools.reduce(operator.add, map(collections.Counter, dict1))

Answer 6

In Python 2.7 you can replace the dict with a collections.Counter object. This supports addition and subtraction of Counters.