Is there any pythonic way to combine two dicts (ad

Question

For example I have two dicts:

Dict A: {'a': 1, 'b': 2, 'c': 3}
Dict B: {'b': 3, 'c': 4, 'd': 5}

I need a pythonic way of 'combining' two dicts such that the result is:

{'a': 1, 'b': 5, 'c': 7, 'd': 5}

That is to say: if a key appears in both dicts, add their values, if it appears in only one dict, keep its value.

Answer 1

myDict = {}
for k in itertools.chain(A.keys(), B.keys()):
    myDict[k] = A.get(k, 0)+B.get(k, 0)

Answer 2

For a more generic and extensible way check mergedict. It uses singledispatch and can merge values based on its types.

Example:

from mergedict import MergeDict

class SumDict(MergeDict):
    @MergeDict.dispatch(int)
    def merge_int(this, other):
        return this + other

d2 = SumDict({'a': 1, 'b': 'one'})
d2.merge({'a':2, 'b': 'two'})

assert d2 == {'a': 3, 'b': 'two'}

Answer 3

Additionally, please note a.update( b ) is 2x faster than a + b

from collections import Counter
a = Counter({'menu': 20, 'good': 15, 'happy': 10, 'bar': 5})
b = Counter({'menu': 1, 'good': 1, 'bar': 3})

%timeit a + b;
## 100000 loops, best of 3: 8.62 µs per loop
## The slowest run took 4.04 times longer than the fastest. This could mean that an intermediate result is being cached.

%timeit a.update(b)
## 100000 loops, best of 3: 4.51 µs per loop

Answer 4

The above solutions are great for the scenario where you have a small number of Counters. If you have a big list of them though, something like this is much nicer:

from collections import Counter

A = Counter({'a':1, 'b':2, 'c':3})
B = Counter({'b':3, 'c':4, 'd':5}) 
C = Counter({'a': 5, 'e':3})
list_of_counts = [A, B, C]

total = sum(list_of_counts, Counter())

print(total)
# Counter({'c': 7, 'a': 6, 'b': 5, 'd': 5, 'e': 3})

The above solution is essentially summing the Counters by:

total = Counter()
for count in list_of_counts:
    total += count
print(total)
# Counter({'c': 7, 'a': 6, 'b': 5, 'd': 5, 'e': 3})

This does the same thing but I think it always helps to see what it is effectively doing underneath.

Answer 5

Merging three dicts a,b,c in a single line without any other modules or libs

If we have the three dicts

a = {"a":9}
b = {"b":7}
c = {'b': 2, 'd': 90}

Merge all with a single line and return a dict object using

c = dict(a.items() + b.items() + c.items())

Returning

{'a': 9, 'b': 2, 'd': 90}

Answer 6

This is a simple solution for merging two dictionaries where += can be applied to the values, it has to iterate over a dictionary only once, I'm surprised no one has suggested this

a = {'a':1, 'b':2, 'c':3}

dicts = [{'b':3, 'c':4, 'd':5},
         {'c':9, 'a':9, 'd':9}]

def merge_dicts(merged,mergedfrom):
    for k,v in mergedfrom.items():
        if k in merged:
            merged[k] += v
        else:
            merged[k] = v
    return merged

for dct in dicts:
    a = merge_dicts(a,dct)
print (a)
#{'c': 16, 'b': 5, 'd': 14, 'a': 10}