Python Cookbook

Searching and Replacing Text in a File

Credit: Jeff Bauer

Problem

You need to change one string into another throughout a file.

Solution

String substitution is most simply performed by the replace method of string objects. The work here is to support reading from the specified file (or standard input) and writing to the specified file (or standard output):

#!/usr/bin/env python
import os, sys

nargs = len(sys.argv)

if not 3 <= nargs <= 5:
    print "usage: %s search_text replace_text [infile [outfile]]" % \
        os.path.basename(sys.argv[0])
else:
    stext = sys.argv[1]
    rtext = sys.argv[2]
    input = sys.stdin
    output = sys.stdout
    if nargs > 3:
        input = open(sys.argv[3])
    if nargs > 4:
        output = open(sys.argv[4], 'w')
    for s in input.xreadlines(  ):
        output.write(s.replace(stext, rtext))
    output.close(  )
    input.close(  )

Discussion

This recipe is really simple, but that’s what beautiful about it—why do complicated stuff when simple stuff suffices? The recipe is a simple main script, as indicated by the leading “shebang” line. The script looks at its arguments to determine the search text, the replacement text, the input file (defaulting to standard input), and the output file (defaulting to standard output). Then, it loops over each line of the input file, writing to the output file a copy of the line with the substitution performed on it. That’s all! For accuracy, it closes both files at the end.

As long as it fits comfortably in memory in two copies (one before and one after the replacement, since strings ...