How Can I Modify Lines in Files In-Place Using Python?

Modifying Lines in Files In-Place

Parsing files line by line provides a valuable capability for manipulating content. However, if the need arises to edit lines within the file as you traverse them, you may wonder if this is achievable.

Inline Editing

Traditionally, modifying files in-place has not been a straightforward task. However, a simulation technique utilizing backup files, similar to the approach taken by Python's fileinput module, can provide a solution.

Example Script

Consider this example script:

#!/usr/bin/env python
import fileinput

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

In this script, we read from files provided on the command line or stdin using fileinput.input. By specifying inplace=True, we enable the modification of the current file. The backup parameter creates a backup file with a .bak extension.

Operation

As the script iterates through each line, it evaluates a given condition (some_condition) on each line. If the condition is met, the line is printed back to the current file, effectively modifying it in-place.

Example Usage

For instance, running the script with:

$ python grep_some_condition.py first_file.txt second_file.txt

will result in first_file.txt and second_file.txt containing only lines that satisfy the some_condition() predicate.

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