failing to print not matched lines from second file

[codinglearner]

Build a set of strings then read the second file - this will fix the problem that you are facing.

with open(DIR/'lines_to_look_for.txt', 'r') as file:
    lines = set([line.strip().split(',')[3] for line in file])
 
with open(DIR/'check_for_lines.txt', 'r') as file:
    for line in file:
        line = line.strip()
        if line.split(',')[3] in lines:
            print(f'{line} MATCH')
        else:
            print(f'{line} NO MATCH')

Square Brackets is list comprehension and the [3] is string slicing. menator01 said right only.