Search and replace a line in a file in Python


Question

I want to loop over the contents of a text file and do a search and replace on some lines and write the result back to the file. I could first load the whole file in memory and then write it back, but that probably is not the best way to do it.

What is the best way to do this, within the following code?

f = open(file)
for line in f:
    if line.contains('foo'):
        newline = line.replace('foo', 'bar')
        # how to write this newline back to the file
1
261
9/7/2009 10:11:09 AM

Accepted Answer

I guess something like this should do it. It basically writes the content to a new file and replaces the old file with the new file:

from tempfile import mkstemp
from shutil import move
from os import fdopen, remove

def replace(file_path, pattern, subst):
    #Create temp file
    fh, abs_path = mkstemp()
    with fdopen(fh,'w') as new_file:
        with open(file_path) as old_file:
            for line in old_file:
                new_file.write(line.replace(pattern, subst))
    #Remove original file
    remove(file_path)
    #Move new file
    move(abs_path, file_path)
177
6/14/2017 4:24:44 PM

The shortest way would probably be to use the fileinput module. For example, the following adds line numbers to a file, in-place:

import fileinput

for line in fileinput.input("test.txt", inplace=True):
    print "%d: %s" % (fileinput.filelineno(), line),

What happens here is:

  1. The original file is moved to a backup file
  2. The standard output is redirected to the original file within the loop
  3. Thus any print statements write back into the original file

fileinput has more bells and whistles. For example, it can be used to automatically operate on all files in sys.args[1:], without your having to iterate over them explicitly. Starting with Python 3.2 it also provides a convenient context manager for use in a with statement.


While fileinput is great for throwaway scripts, I would be wary of using it in real code because admittedly it's not very readable or familiar. In real (production) code it's worthwhile to spend just a few more lines of code to make the process explicit and thus make the code readable.

There are two options:

  1. The file is not overly large, and you can just read it wholly to memory. Then close the file, reopen it in writing mode and write the modified contents back.
  2. The file is too large to be stored in memory; you can move it over to a temporary file and open that, reading it line by line, writing back into the original file. Note that this requires twice the storage.

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