"Adding" Dictionaries in Python? [duplicate]

Question

How about that:

dict( [ (n, a.get(n, 0)+b.get(n, 0)) for n in set(a)|set(b) ] )

Or without creating an intermediate list (generator is enough):

dict( (n, a.get(n, 0)+b.get(n, 0)) for n in set(a)|set(b) )

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Post Scriptum:

As a commentator addressed correctly, there is a way to implement that easier with the new (from Py2.7) collections.Counter class. As much I remember, this version was not available when I wrote the answer:

from collections import Counter
dict(Counter(a)+Counter(b))

Not in one line, but ...

import itertools
import collections
a = dict()
a['cat'] = 1
a['fish'] = 10
a['aardvark'] = 1000
b = dict()
b['cat'] = 2
b['dog'] = 200
b['aardvark'] = 2000
c = collections.defaultdict(int)
for k, v in itertools.chain(a.iteritems(), b.iteritems()):
    c[k] += v

You can easily extend it to a larger number of dictionaries.

result in a:

for elem in b:
    a[elem] = a.get(elem, 0) + b[elem]

result in c:

c = dict(a)
for elem in b:
    c[elem] = a.get(elem, 0) + b[elem]

One liner (as sortof requested): get key lists, add them, discard duplicates, iterate over result with list comprehension, return (key,value) pairs for the sum if the key is in both dicts, or just the individual values if not. Wrap in dict.

>>> dict([(x,a[x]+b[x]) if (x in a and x in b) else (x,a[x]) if (x in a) else (x,b[x]) for x in set(a.keys()+b.keys())])
{'aardvark': 3000, 'fish': 10, 'dog': 200, 'cat': 3}