How Can I Edit File Lines In-Place Using Python?

Editing File Lines In-Place

It is not possible to directly edit a line in-place while parsing a file line by line using standard Python functions. However, it is possible to simulate in-place editing using a backup file.

The fileinput Module

The fileinput module provides a way to simulate in-place editing. It works by:

• Creating a backup file with the extension specified by the backup parameter (default: '.bak').
• Reading from the backup file instead of the original file.
• Writing to the original file when the inplace parameter is set to True.

In-Place Line Editing Example

Here's an example script that removes lines that do not satisfy a some_condition predicate from files specified on the command line or stdin:

#!/usr/bin/env python
# grep_some_condition.py
import fileinput

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

Usage:

$ python grep_some_condition.py first_file.txt second_file.txt

After running this script, first_file.txt and second_file.txt will only contain lines that satisfy the some_condition() predicate.

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