Merging dictionaries

Question

I need to append the values from one dictionary (N) to another (M) - pseudocode below

if[x] in M:
  M[x]=M[x]+N[x]
else:
  M[x]=N[x] 

Doing this for every key in N seems quite untidy coding.
What would be the most efficient way to achieve this?

Answer 1

Of course you should be iterating your keys in "x" already - but a single line solution is:

M.update({key:((M[key] + value) if key in M else value) for key, value in N.items()})

Answer 2

not entirely sure what your x is (guessing the keys of both M and N), then this might work:

M = {key: M.get(key, 0) + N.get(key, 0) for key in set((*M, *N))}

for the example:

M = {'a': 1, 'c': 3, 'e': 5}
N = {'a': 2, 'b': 4, 'c': 6, 'd': 8}

you get:

print(M)  # {'a': 3, 'e': 5, 'd': 8, 'c': 9, 'b': 4}

or please clarify what the desired output for the given example would be.

Answer 3

When you say "append", I assume that means that the values in your dicts are lists. However the techniques below can easily be adapted if they're simple objects like integers or strings.

Python doesn't provide any built-in methods for handling dicts of lists, and the most efficient way to do this depends on the data, because some ways work best when there are a high proportion of shared keys, other ways are better when there aren't many shared keys.

If the proportion of shared keys isn't too high, this code is reasonably efficient:

m = {1:[1, 2], 2:[3]}
n = {1:[4], 3:[5]}

for k, v in n.items():
    m.setdefault(k, []).extend(v)

print(m)

output

{1: [1, 2, 4], 2: [3], 3: [5]}

You can make this slightly faster by caching the .setdefault method and the empty list:

def merge(m, n):
    setdef = m.setdefault
    empty = []
    for k, v in n.items():
        setdef(k, empty).extend(v)

If you expect a high proportion of shared keys, then it's better to perform set operations on the keys (in Python 3, dict.keys() return a set-like View object, which is extremely efficient to construct), and handle the shared keys separately from the unique keys of N.