How can I change one line in text file with python

Question

I have a txt file that I need to read and find a line, and change its value inside the same file. I am using Python.

for file in os.listdir(path2):
    if file.startswith('nasfla.in'):
        crack_in = ''.join((path2, '\\', file))
        file_in = open(crack_in, 'r')
        with file_in:
            for line in file_in:
                #looking for the line that I need to change
                if (str(line[1:11])) == 'schedcount':
                # change the line with new value

I would like to change what is in the line that starts with 'schedcount' but I don't know how to read and write in file in same time.

Thanks!

Possible duplicate of Read/Write text file

Mat
– Mat

2019-03-06 19:49:43 +00:00
Commented Mar 6, 2019 at 19:49 — Mat
– Mat, Commented Mar 6, 2019 at 19:49

Merig · Accepted Answer · 2019-03-06 19:09:02Z

3

Updating lines while iterating is tricky, you're better off rewriting the file if it's not too big:

with open('old_file', 'r') as input_file, open('new_file', 'w') as output_file:
    for line in input_file:
        if 'schedcount' in line:
            output_file.write('new line\n')
        else:
            output_file.write(line)

answered Mar 6, 2019 at 19:09

Merig

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3 Comments

salah Over a year ago

just one question there is no way that I can keep the same name of file because I need the file as an input for a subprocess and if I change the file name the subprocess will stop

Merig Over a year ago

You can use os.rename('new_file', 'old_file') after the loop

salah Over a year ago

Just to reminde people before you rename the new file with name of the old file dont forget to remove the old file from the directory or it will fail use os.remove(old file)

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