How Can I Modify File Lines In-Place Efficiently?

Modifying Lines in a File In-Place

Can you parse a file line by line and edit lines on the go?

Yes, it is possible to simulate in-place editing using a backup file, similar to stdlib's fileinput module's approach.

Consider the following script:

import fileinput

for line in fileinput.input(inplace=True, backup='.bak'):
    if some_condition(line):
        print(line, end='')

This script removes lines from the specified files that do not meet a certain condition, redirecting the modified content back into the original files.

For instance, to filter lines based on a condition in files first_file.txt and second_file.txt:

python grep_some_condition.py first_file.txt second_file.txt

After execution, first_file.txt and second_file.txt will contain only lines that satisfy the some_condition().

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