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

fileinput module has very ugly API, I find beautiful module for this task - in_place, example for Python 3:

import in_place

with in_place.InPlace('data.txt') as file:
    for line in file:
        line = line.replace('test', 'testZ')
        file.write(line)

main difference from fileinput:

• Instead of hijacking sys.stdout, a new filehandle is returned for writing.
• The filehandle supports all of the standard I/O methods, not just readline().

Important Notes:

1. This solution deletes every line in the file if you don't re-write it with the file.write() line.
2. Also, if the process is interrupted, you lose any line in the file that has not already been re-written.

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.