merge two dicts with custom update

Question

I have two Python dictionaries, and I want to merge (union) them together but with a custom update function in case the same key exists in both dicts. For instance:

>>> x = {'a': 1, 'b': 2}
>>> y = {'b': 10, 'c': 11}
>>> z = {'a': 3, 'd': 5}
>>> merge(x,y,z)  # add values together if key exists in both
{'a': 4, 'b': 12, 'c': 11, 'd': 5}

What's the simplest way to do so?

Edit: I'm looking for a solution that works for arbitrary number of dicts to merge, and perform arbitrary operation when the same key is in more than 1 dict (not necessarily add).

[Check this out](https://stackoverflow.com/questions/11011756/is-there-any-pythonic-way-to-combine-two-dicts-adding-values-for-keys-that-appe) — , Jun 18 '21 at 04:17
Using `collections.Counter`, you can convert dictionaries to Counter objects, add them and then recast the result as dictionary. — Zoro, Jun 18 '21 at 04:19
Does this answer your question? [Is there any pythonic way to combine two dicts (adding values for keys that appear in both)?](https://stackoverflow.com/questions/11011756/is-there-any-pythonic-way-to-combine-two-dicts-adding-values-for-keys-that-appe) — Stuti Rastogi, Jun 18 '21 at 04:22

score 3 · Answer 1 · answered Jun 18 '21 at 04:16

3

Use collections.Counter. It will help you.

from collections import Counter
A = Counter({'a': 1, 'b': 2})
B = Counter({'b': 10, 'c': 11})
print(dict(A+B))

Output

{'a': 1, 'b': 12, 'c': 11}

answered Jun 18 '21 at 04:16

ThePyGuy · Answer 2 · 2021-06-18T04:56:35.767

Get the union of all the keys in both the dictionary, and just add values from both the dictionary using dict.get() with default value as 0, you can use dictionary-comprehension.

def merge(d1, d2):
    keys = set(d1.keys()).union(d2.keys())
    return {k: d1.get(k,0)+d2.get(k,0) for k in keys}

OUTPUT:

>>> merge(x, y)
{'c': 11, 'a': 1, 'b': 12}

SPEED COMPARISON (>4X Faster):

using merge (custom function): 
2.99 µs ± 574 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

using Counter: 
14.2 µs ± 709 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

After UPDATE to the question :

You can just modify the function a bit using *args if you want to do it for any number of dictionaryies:

def merge(*args):
    keys = set()
    for arg in args:
        keys = keys.union(arg.keys())
    return {k: sum(arg.get(k,0) for arg in args) for k in keys}

OUTPUT:

>>> merge(x, y, z)
{'a': 4, 'd': 5, 'c': 11, 'b': 12}

How would this work if I have a arbitrary number of dicts to merge? — JRR, Jun 18 '21 at 04:29

merge two dicts with custom update

2 Answers2