Search and replace a line in a file in Python

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Search and replace a line in a file in Python

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
Asked By: pkit


Answer #1:

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, copymode
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))
    #Copy the file permissions from the old file to the new file
    copymode(file_path, abs_path)
    #Remove original file
    #Move new file
    move(abs_path, file_path)
Answered By: Thomas Watnedal

Answer #2:

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('{} {}'.format(fileinput.filelineno(), line), end='') # for Python 3
    # print "%d: %s" % (fileinput.filelineno(), line), # for Python 2

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.
Answered By: Eli Bendersky

Answer #3:

Here’s another example that was tested, and will match search & replace patterns:

import fileinput
import sys
def replaceAll(file,searchExp,replaceExp):
    for line in fileinput.input(file, inplace=1):
        if searchExp in line:
            line = line.replace(searchExp,replaceExp)

Example use:

Answered By: Jason

Answer #4:

This should work: (inplace editing)

import fileinput
# Does a list of files, and
# redirects STDOUT to the file in question
for line in fileinput.input(files, inplace = 1):
      print line.replace("foo", "bar"),
Answered By: Kinlan

Answer #5:

Based on the answer by Thomas Watnedal.
However, this does not answer the line-to-line part of the original question exactly. The function can still replace on a line-to-line basis

This implementation replaces the file contents without using temporary files, as a consequence file permissions remain unchanged.

Also re.sub instead of replace, allows regex replacement instead of plain text replacement only.

Reading the file as a single string instead of line by line allows for multiline match and replacement.

import re
def replace(file, pattern, subst):
    # Read contents from file as a single string
    file_handle = open(file, 'r')
    file_string =
    # Use RE package to allow for replacement (also allowing for (multiline) REGEX)
    file_string = (re.sub(pattern, subst, file_string))
    # Write contents to file.
    # Using mode 'w' truncates the file.
    file_handle = open(file, 'w')
Answered By: Thijs

Answer #6:

As lassevk suggests, write out the new file as you go, here is some example code:

fin = open("a.txt")
fout = open("b.txt", "wt")
for line in fin:
    fout.write( line.replace('foo', 'bar') )
Answered By: hamishmcn

Answer #7:

A more pythonic way would be to use context managers like the code below:

from tempfile import mkstemp
from shutil import move
from os import remove
def replace(source_file_path, pattern, substring):
    fh, target_file_path = mkstemp()
    with open(target_file_path, 'w') as target_file:
        with open(source_file_path, 'r') as source_file:
            for line in source_file:
                target_file.write(line.replace(pattern, substring))
    move(target_file_path, source_file_path)

You can find the full snippet here.

Answered By: Kiran

Answer #8:

If you’re wanting a generic function that replaces any text with some other text, this is likely the best way to go, particularly if you’re a fan of regex’s:

import re
def replace( filePath, text, subs, flags=0 ):
    with open( filePath, "r+" ) as file:
        fileContents =
        textPattern = re.compile( re.escape( text ), flags )
        fileContents = textPattern.sub( subs, fileContents ) 0 )
        file.write( fileContents )
Answered By: starryknight64

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