# How to sum dict elements

Posted on

### Question :

How to sum dict elements

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.

A little ugly, but a one-liner:

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

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))
``````

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}
``````

``````>>> {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

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}
``````

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 đź™‚

I was interested in the performance of the proposed Counter, reduce and sum methods for large lists. Maybe someone else is interested in this as well.
You can have a look here: https://gist.github.com/torstenrudolf/277e98df296f23ff921c

I tested the three methods for this list of dictionaries:

``````dictList = [{'a': x, 'b': 2*x, 'c': x**2} for x in xrange(10000)]
``````

the sum method showed the best performance, followed by reduce and Counter was the slowest. The time showed below is in seconds.

``````In [34]: test(dictList)
Out[34]:
{'counter': 0.01955194902420044,
'reduce': 0.006518083095550537,
'sum': 0.0018319153785705566}
``````

But this is dependent on the number of elements in the dictionaries. the sum method will slow down faster than the reduce.

``````l = [{y: x*y for y in xrange(100)} for x in xrange(10000)]

In [37]: test(l, num=100)
Out[37]:
{'counter': 0.2401433277130127,
'reduce': 0.11110662937164306,
'sum': 0.2256883692741394}
``````

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

Here is a reasonable beatiful one.

``````final = {}
for k in dict1[0].Keys():
final[k] = sum(x[k] for x in dict1)
return final
``````