Modifying Lines in a File In-Place

Is it possible to efficiently parse a file line by line and modify specific lines without creating a copy or temporary file?

Answer:

Yes, it is possible to simulate in-place editing using a backup file. The fileinput module provides the necessary functionality for this.

Example:

Consider a script that removes lines not meeting a certain condition from files provided as command-line arguments or stdin:

import fileinput

for line in fileinput.input(inplace=True, backup='.bak'):
    if some_condition(line):
        print(line)  # The modified line is output to the current file

Usage:

To remove lines not satisfying some_condition from first_file.txt and second_file.txt, run the following command:

python grep_some_condition.py first_file.txt second_file.txt

Upon completion, first_file.txt and second_file.txt will only contain lines that meet the some_condition() predicate.

This approach ensures efficient in-place modification by creating a backup file .bak during the editing process, preserving the original file if any errors occur.