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

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
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)

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 "%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.

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)
        sys.stdout.write(line)

Example use:

replaceAll("/fooBar.txt","Hello\sWorld!$","Goodbye\sWorld.")

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"),

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 = file_handle.read()
    file_handle.close()

    # 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')
    file_handle.write(file_string)
    file_handle.close()

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') )
fin.close()
fout.close()

Answer 7

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 = file.read()
        textPattern = re.compile( re.escape( text ), flags )
        fileContents = textPattern.sub( subs, fileContents )
        file.seek( 0 )
        file.truncate()
        file.write( fileContents )

Answer 8

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))
    remove(source_file_path)
    move(target_file_path, source_file_path)

You can find the full snippet here.

Answer 9

Create a new file, copy lines from the old to the new, and do the replacing before you write the lines to the new file.

Answer 10

Expanding on @Kiran's answer, which I agree is more succinct and Pythonic, this adds codecs to support the reading and writing of UTF-8:

import codecs 

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 codecs.open(target_file_path, 'w', 'utf-8') as target_file:
        with codecs.open(source_file_path, 'r', 'utf-8') as source_file:
            for line in source_file:
                target_file.write(line.replace(pattern, substring))
    remove(source_file_path)
    move(target_file_path, source_file_path)

Answer 11

Using hamishmcn's answer as a template I was able to search for a line in a file that match my regex and replacing it with empty string.

import re 

fin = open("in.txt", 'r') # in file
fout = open("out.txt", 'w') # out file
for line in fin:
    p = re.compile('[-][0-9]*[.][0-9]*[,]|[-][0-9]*[,]') # pattern
    newline = p.sub('',line) # replace matching strings with empty string
    print newline
    fout.write(newline)
fin.close()
fout.close()

Answer 12

if you remove the indent at the like below, it will search and replace in multiple line. See below for example.

def replace(file, pattern, subst):
    #Create temp file
    fh, abs_path = mkstemp()
    print fh, abs_path
    new_file = open(abs_path,'w')
    old_file = open(file)
    for line in old_file:
        new_file.write(line.replace(pattern, subst))
    #close temp file
    new_file.close()
    close(fh)
    old_file.close()
    #Remove original file
    remove(file)
    #Move new file
    move(abs_path, file)

Answer 13

For Linux users:

import os
os.system('sed -i \'s/foo/bar/\' '+file_path)