Merge two dictionaries and keep the values for duplicate keys in Python

Question

Let\'s suppose that I have two dictionaries:

dic1 =  { \"first\":1, \"second\":4, \"third\":8} 
dic2 =  { \"first\":9, \"second\":5, \"fourth\":3}

Answer 1

Here's a naive solution; copy one of the dictionaries over to the result and iterate over the other dictionary's keys and values, adding lists to the result as necessary. Since there are only two dictionaries, no merged list will have more than 2 items.

dic1 = {"first": 1, "second": 4, "third": 8} 
dic2 = {"first": 9, "second": 5, "fourth": 3}
dic3 = dict(dic2)

for k, v in dic1.items():
    dic3[k] = [dic3[k], v] if k in dic3 else v

print(dic3) # => {'first': [9, 1], 'second': [5, 4], 'fourth': 3, 'third': 8}

If you'd like single values to be lists (likely better design; mixed types aren't much fun to deal with) you can use:

dic3 = {k: [v] for k, v in dic2.items()}

for k, v in dic1.items():
    dic3[k] = dic3[k] + [v] if k in dic3 else [v]

print(dic3) # => {'first': [9, 1], 'second': [5, 4], 'fourth': [3], 'third': [8]}

---

Generalizing it to any number of dictionaries:

def merge_dicts(*dicts):
    """
    >>> merge_dicts({"a": 2}, {"b": 4, "a": 3}, {"a": 1})
    {'a': [2, 3, 1], 'b': [4]}
    """
    merged = {}
    
    for d in dicts:
        for k, v in d.items():
            if k not in merged:
                merged[k] = []

            merged[k].append(v)
    
    return merged

You can use collections.defaultdict to clean it up a bit if you don't mind the import:

from collections import defaultdict

def merge_dicts(*dicts):
    """
    >>> merge_dicts({"a": 2}, {"b": 4, "a": 3}, {"a": 1})
    defaultdict(, {'a': [2, 3, 1], 'b': [4]})
    """
    merged = defaultdict(list)
    
    for d in dicts:
        for k, v in d.items():
            merged[k].append(v)
    
    return merged