How can I separate a .json file by keys stored in a different .json file?

I have two files, one containing this type of data:

{"v_uqiMw7tQ1Cc": {"duration": 55.15, "timestamps": [[0.28, 55.15], [13.79, 54.32]]}, "v_bXdq2zI1Ms0": {"duration": 73.1, "timestamps": [[0, 10.23], [10.6, 39.84], [38.01, 73.1]]}
    

and the other file contains the splits ids:

{train_set:{"v_uqiMw7tQ1Cc","v_uqiMfergQ1Cc"}, test_set:{"v_bXdq2zI1Ms0", "v_bXdfreht2Ms0"}}

How do I change the first file for it to have the subset it belongs to?:

{"v_uqiMw7tQ1Cc": {"duration": 55.15, subset: "train", "timestamps": [[0.28, 55.15], [13.79, 54.32]]}, "v_bXdq2zI1Ms0": {"duration": 73.1, subset: "test", "timestamps": [[0, 10.23], [10.6, 39.84], [38.01, 73.1]]}

Try below code, I guess this is what you are looking for

import json

with open('json_file/json_set1.json') as f1:
    file1 = json.load(f1)
with open('json_file/json_set2.json') as f2:
    file2 = json.load(f2)

print(file1) 

{'v_uqiMw7tQ1Cc': {'duration': 55.15, 'timestamps': [[0.28, 55.15], [13.79, 54.32]]}, 'v_bXdq2zI1Ms0': {'duration': 73.1, 'timestamps': [[0, 10.23], [10.6, 39.84], [38.01, 73.1]]}}

print(file2)

{'train_set': ['v_uqiMw7tQ1Cc', 'v_uqiMfergQ1Cc'], 'test_set': ['v_bXdq2zI1Ms0', 'v_bXdfreht2Ms0']}

for key1, val2 in file1.items():
    for key2, val2 in file2.items():
        if key1 in file2[key2]:
            file1[key1].update({'subset': key2})

print(file1)

{'v_uqiMw7tQ1Cc': {'duration': 55.15, 'timestamps': [[0.28, 55.15], [13.79, 54.32]], 'subset': 'train_set'},

'v_bXdq2zI1Ms0': {'duration': 73.1, 'timestamps': [[0, 10.23], [10.6, 39.84], [38.01, 73.1]], 'subset': 'test_set'}}