Python: Elegantly merge dictionaries with sum() of values

Question

I'm trying to merge logs from several servers. Each log is a list of tuples (date, count). date may appear more than once, and I want the resulting dictionary to hold the sum of all counts from all servers.

Here's my attempt, with some data for example:

from collections import defaultdict

a=[("13.5",100)]
b=[("14.5",100), ("15.5", 100)]
c=[("15.5",100), ("16.5", 100)]
input=[a,b,c]

output=defaultdict(int)
for d in input:
        for item in d:
           output[item[0]]+=item[1]
print dict(output)

Which gives:

{'14.5': 100, '16.5': 100, '13.5': 100, '15.5': 200}

As expected.

I'm about to go bananas because of a colleague who saw the code. She insists that there must be a more Pythonic and elegant way to do it, without these nested for loops. Any ideas?

Answer 1

Doesn't get simpler than this, I think:

a=[("13.5",100)]
b=[("14.5",100), ("15.5", 100)]
c=[("15.5",100), ("16.5", 100)]
input=[a,b,c]

from collections import Counter

print sum(
    (Counter(dict(x)) for x in input),
    Counter())

Note that Counter (also known as a multiset) is the most natural data structure for your data (a type of set to which elements can belong more than once, or equivalently - a map with semantics Element -> OccurrenceCount. You could have used it in the first place, instead of lists of tuples.

---

Also possible:

from collections import Counter
from operator import add

print reduce(add, (Counter(dict(x)) for x in input))

Using reduce(add, seq) instead of sum(seq, initialValue) is generally more flexible and allows you to skip passing the redundant initial value.

Note that you could also use operator.and_ to find the intersection of the multisets instead of the sum.

---

The above variant is terribly slow, because a new Counter is created on every step. Let's fix that.

We know that Counter+Counter returns a new Counter with merged data. This is OK, but we want to avoid extra creation. Let's use Counter.update instead:

update(self, iterable=None, **kwds) unbound collections.Counter method

Like dict.update() but add counts instead of replacing them. Source can be an iterable, a dictionary, or another Counter instance.

That's what we want. Let's wrap it with a function compatible with reduce and see what happens.

def updateInPlace(a,b):
    a.update(b)
    return a

print reduce(updateInPlace, (Counter(dict(x)) for x in input))

This is only marginally slower than the OP's solution.

Benchmark: http://ideone.com/7IzSx (Updated with yet another solution, thanks to astynax)

(Also: If you desperately want an one-liner, you can replace updateInPlace by lambda x,y: x.update(y) or x which works the same way and even proves to be a split second faster, but fails at readability. Don't :-))

Answer 2

from collections import Counter


a = [("13.5",100)]
b = [("14.5",100), ("15.5", 100)]
c = [("15.5",100), ("16.5", 100)]

inp = [dict(x) for x in (a,b,c)]
count = Counter()
for y in inp:
  count += Counter(y)
print(count)

output:

Counter({'15.5': 200, '14.5': 100, '16.5': 100, '13.5': 100})

Edit: As duncan suggested you can replace these 3 lines with a single line:

   count = Counter()
    for y in inp:
      count += Counter(y)

replace by : count = sum((Counter(y) for y in inp), Counter())

Answer 3

You could use itertools' groupby:

from itertools import groupby, chain

a=[("13.5",100)]
b=[("14.5",100), ("15.5", 100)]
c=[("15.5",100), ("16.5", 100)]
input = sorted(chain(a,b,c), key=lambda x: x[0])

output = {}
for k, g in groupby(input, key=lambda x: x[0]):
  output[k] = sum(x[1] for x in g)

print output

The use of groupby instead of two loops and a defaultdict will make your code clearer.

Answer 4

You can use Counter or defaultdict, or you can try my variant:

def merge_with(d1, d2, fn=lambda x, y: x + y):
    res = d1.copy() # "= dict(d1)" for lists of tuples
    for key, val in d2.items(): # ".. in d2" for lists of tuples
        try:
            res[key] = fn(res[key], val)
        except KeyError:
            res[key] = val
    return res

>>> merge_with({'a':1, 'b':2}, {'a':3, 'c':4})
{'a': 4, 'c': 4, 'b': 2}

Or even more generic:

def make_merger(fappend=lambda x, y: x + y, fempty=lambda x: x):
    def inner(*dicts):
        res = dict((k, fempty(v)) for k, v
            in dicts[0].items()) # ".. in dicts[0]" for lists of tuples
        for dic in dicts[1:]:
            for key, val in dic.items(): # ".. in dic" for lists of tuples
                try:
                    res[key] = fappend(res[key], val)
                except KeyError:
                    res[key] = fempty(val)
        return res
    return inner

>>> make_merger()({'a':1, 'b':2}, {'a':3, 'c':4})
{'a': 4, 'c': 4, 'b': 2}

>>> appender = make_merger(lambda x, y: x + [y], lambda x: [x])
>>> appender({'a':1, 'b':2}, {'a':3, 'c':4}, {'b':'BBB', 'c':'CCC'})
{'a': [1, 3], 'c': [4, 'CCC'], 'b': [2, 'BBB']}

Also you can subclass the dict and implement a __add__ method: