How do I compare every line of the 1st text file to every line of the 2nd text file in Python?

I have 2 text files named f1 & f2 with 100k lines of names each. I want to compare the first line of f1 with every line of f2, then the second line of f1 with every line of f2, and so on. I already tried using nested for loop like code below but it doesn’t work.

What am I doing wrong I can’t seem to find? Please can someone tell me?

Thanks in advance.

old.txt

sourcreameggnest
saturnnixgreentea
saxophonedesertham
footballplumvirgo
soybeansthesting
cauliflowertornado
sourcreameggnest
saturnnixgreentea

new.txt

goldfishpebbleduck
saxophonedesertham
footballplumvirgo
abloomtheavengers
venisonflowersea
goodfellaswalker
saturnnixgreentea

Code:

 with open('old.txt', 'r') as f1, open('new.txt', 'r') as f2:
    
    for line1 in f1:
        print('Line 1:- ' + line1, end='')
        
        for line2 in f2:
            print('Line 2:- ' + line2, end='')
            
            if line1.strip() == line2:
                print("Inside comparison" + line1, end='')

Output:

Line 1:- goldfishpebbleduck
Line 2:- sourcreameggnest
Line 2:- saturnnixgreentea
Line 2:- saxophonedesertham
Line 2:- footballplumvirgo
Line 2:- soybeansthesting
Line 2:- cauliflowertornado
Line 2:- sourcreameggnest
Line 2:- saturnnixgreentea
Line 1:- saxophonedesertham
Line 1:- footballplumvirgo
Line 1:- abloomtheavengers
Line 1:- venisonflowersea
Line 1:- goodfellaswalker
Line 1:- saturnnixgreentea

>Solution :

Combining the answers of @LukasNeugebauer and @Thierry Lathuille, here’s what your code should look like:

with open('old.txt', 'r') as f1, open('new.txt', 'r') as f2:
    lines1 = f1.readlines()
    lines2 = f2.readlines()
    for line1 in lines1:
        print('Line 1:- ' + line1, end='')
        if line1 in lines2:
            print("Inside comparison" + line1, end='')

If you are wondering, whether using in check is faster then iterating through the second list and comparing each value with ==, I tested it. For both files containing 10,000 lines of random strings, it took ~2.8 seconds to process them fully with two loops and only ~0.8 using the in operator.

If your files are not bigger than a megabyte, I wouldn’t really bother optimizing this, but otherwise you should really think about what you are actually comparing and what shortcuts can you use.