select the first of two or more filenames and save only the first

October 11, 2023

I want to save files with these filenames:

filenames = [
    '4_66\nNUC_66377\nAPPR\nWONDER',
    '4_66\nNUC_66377\nAPPR\nCOT',
    '8_21\nAKRO\nNUT\nAMY'
    ]

and if the filename starts with the same number before underscore, then write to file only the first filename.

So, I am doing:

for idx in range(len(filenames)-1):
    if filenames[idx][0:2] != filenames[idx + 1][0:2]:
        with open('./' + filenames[idx] + '.txt', 'w') as file:
            file.write('111')

# save the last file
with open('./' + filenames[-1] + '.txt', 'w') as file:
        file.write('111')

But if you run the code, it saves the second one!
'4_66\nNUC_66377\nAPPR\nCOT'

>Solution :

Rely on set containment check to only process the files with encountered unique custom prefix:

fn_set = set()
for f in filenames:
    pfx = f[:f.find('_')]  # prefix before underscore
    if pfx not in fn_set:
        f.write('yourdata')
        fn_set.add(pfx)