fastest way to achieve this in python? (simple)

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i have list of user:pass, and list of users

how can I compare the list of users with the user:pass, and write down the ones that match

I can do this, but it is slow for big files

combo = open('combo.txt','r').read().splitlines()
users = open('users.txt','r').read().splitlines()

for line in combo:
    for user in users:
        if user in line


you get what i mean, but doing this is very slow when working with big user and combo lists