如何在不覆盖前一个键的情况下合并列表中的两个字典

Question

For the dictionaries in a list, I need to extract and merge them in a single dictionary without overwriting the existing key values.

For example, I have:

mylist = [{'b': 3}, {'b': 9, 'A': 8, 'Z': 2, 'V': 1}]

The result should be:

{'b': 3, 'A': 8, 'Z': 2, 'V': 1}

Below is my code:

def concatenate_dict(dict_list: list) -> dict:
    final_dict = {}
    for d in dict_list:
        for k, v in d.items():
            if k not in final_dict:
                final_dict |= (k, v)

    return final_dict


mylist = [{'b': 7}, {'b': 10, 'A': 8, 'Z': 2, 'V': 1}]
print(concatenate_dict(mylist))

I do not understand why the "not in" keyword would not skip the existing 'b': 10 item and leave 'b': 7 alone and continue to 'A': 8

Answer 1

Instead of iterating over every dictionary, how about reverse mylist and iteratively update an output dictionary? Then every key-value pair inserted into out from the back will be updated from the front if a key already exists.

out = {}
for d in mylist[::-1]:
    out.update(d)

Output:

{'b': 3, 'A': 8, 'Z': 2, 'V': 1}

Answer 2

Fix

Use the appropriate operator, you can't use |= with a tuple

def concatenate_dict(dict_list: list) -> dict:
    final_dict = {}
    for d in dict_list:
        for k, v in d.items():
            if k not in final_dict:
                final_dict[k] = v
    return final_dict

---

Improve

Iterate on the dicts backwards and use dict.update

def concatenate_dict(dict_list: list) -> dict:
    final_dict = {}
    for d in reversed(dict_list):
        final_dict.update(d)
    return final_dict

Could be with reduce and merge operator

from functools import reduce

def concatenate_dict(dict_list: list) -> dict:
    return reduce(lambda x, y: x | y, reversed(dict_list))

Answer 3

I think the best way to do it, both in terms of readability and efficiency, is to iterate on your dicts in reverse order, as suggested in the Improve section of azro's answer .

However, I'd like to highlight methoddict.setdefault , which does exactly what you want: add a key: value to a dict, only if the key wasn't already in the dict.

def concatenate_dict(dict_list: list) -> dict:
    final_dict = {}
    for d in dict_list:
        for k, v in d.items():
            final_dict.setdefault(k, v)
    return final_dict


mylist = [{'b': 7}, {'b': 10, 'A': 8, 'Z': 2, 'V': 1}]
print(concatenate_dict(mylist))
# {'b': 7, 'A': 8, 'Z': 2, 'V': 1}