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Recipe 2.3. Searching and Replacing Text in a File
Credit: Jeff Bauer, Adam
Krieg
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 a specified file (or standard input) and
writing to a 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_file = sys.stdin
output_file = sys.stdout
if nargs > 3:
7 input_file = open(sys.argv[3])
if nargs > 4:
output_file = open(sys.argv[4], 'w')
for s in input_file:
output_file.write(s.replace(stext, rtext))
output.close( )
input.close( )
Discussion
This recipe is
really simple, but that's what beautiful about
itwhy do complicated stuff when simple stuff suffices? As
indicated by the leading "shebang"
line, the recipe is a simple main script, meaning a script meant to
be run directly at a shell command prompt, as opposed to a module
meant to be imported from elsewhere. 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, the script closes both files at the end.As long as an input file fits comfortably in memory in two copies
(one before and one after the replacement, since strings are
immutable), we could, with an increase in speed, operate on the
entire input file's contents at once instead of
looping. With today's low-end PCs typically
containing at least 256 MB of memory, handling files of up to about
100 MB should not be a problem, and few text files are bigger than
that. It suffices to replace the for loop with one
single statement:
output_file.write(input_file.read( ).replace(stext, rtext))As you can see, that's even simpler than the loop
used in the recipe.
See Also
Documentation for the open built-in function,
file objects, and strings'
replace method in the Library
Reference and Python in a
Nutshell.
