Is it possible to modify lines in a file in-place?

Question

Is it possible to parse a file line by line, and edit a line in-place while going through the lines?

Answer 1

Is it possible to parse a file line by line, and edit a line in-place while going through the lines?

It can be simulated using a backup file as stdlib's fileinput module does.

Here's an example script that removes lines that do not satisfy some_condition from files given 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

Example:

$ python grep_some_condition.py first_file.txt second_file.txt

On completion first_file.txt and second_file.txt files will contain only lines that satisfy some_condition() predicate.