#!/usr/bin/env python
import os
f, o = open('file.txt', 'r'), open('out.txt', 'a')
for line in f.readlines():
    if 'some string' not in line:
        o.write(line)
f.close()
o.close()
os.popen('rm file.txt')
os.popen('mv out.txt file.txt')