replacing characters in existing large file

Question

i have a large file in my local disk which contains some fixed length string in first line. I need to programmatically replace that fixed length string using python without reading whole file in memory .

i have tried opening the file in append mode and seeking to 0 position. And then replace the string which is of 9 bytes. The code is also added here , what i tried .

    with open ("largefile.txt", 'a') as f:
        f.seek(0,0)
        f.write("123456789")

I think this could help you: https://stackoverflow.com/questions/14947238/change-first-line-of-a-file-in-python — mymindwentblvnk, Jun 25 '19 at 08:11
thank you , but i have a very large file which is running in GBs , so creating a duplicate file would be too slow for me . — Parnit Kaur, Jun 25 '19 at 08:18

score 2 · Accepted Answer · answered Jun 25 '19 at 09:07

I think you just want to open the file for writing without truncating it, which would be r+. to make this reproducible, we first create a file that matches this format:

with open('many_lines.txt', 'w') as fd:
    print('abcdefghi', file=fd)
    for i in range(10000):
        print(f'line {i:09}', file=fd)

then we basically do what you were doing, but with the correct mode:

with open('many_lines.txt', 'r+') as fd:
    print('123456789', file=fd)

or you can use write directly, with:

with open('many_lines.txt', 'r+') as fd:
    fd.write('123456789')

Note: I'm opening in r+ so that you'll get an FileNotFoundError if it doesn't exist (or the filename is misspelled) rather than just blindly creating a tiny file

The open modes are directly copied from the C/POSIX API for the fopen so your use of a will trigger behaviour that says:

Subsequent writes to the file will always end up at the then current end of file, irrespective of any intervening fseek(3) or similar

replacing characters in existing large file

1 Answers1