How to modify a text file?

Question

Unfortunately there is no way to insert into the middle of a file without re-writing it. As previous posters have indicated, you can append to a file or overwrite part of it using seek but if you want to add stuff at the beginning or the middle, you'll have to rewrite it.

This is an operating system thing, not a Python thing. It is the same in all languages.

What I usually do is read from the file, make the modifications and write it out to a new file called myfile.txt.tmp or something like that. This is better than reading the whole file into memory because the file may be too large for that. Once the temporary file is completed, I rename it the same as the original file.

This is a good, safe way to do it because if the file write crashes or aborts for any reason, you still have your untouched original file.

Depends on what you want to do. To append you can open it with "a":

 with open("foo.txt", "a") as f:
     f.write("new line\n")

If you want to preprend something you have to read from the file first:

with open("foo.txt", "r+") as f:
     old = f.read() # read everything in the file
     f.seek(0) # rewind
     f.write("new line\n" + old) # write the new line before

The fileinput module of the Python standard library will rewrite a file inplace if you use the inplace=1 parameter:

import sys
import fileinput

# replace all occurrences of 'sit' with 'SIT' and insert a line after the 5th
for i, line in enumerate(fileinput.input('lorem_ipsum.txt', inplace=1)):
    sys.stdout.write(line.replace('sit', 'SIT'))  # replace 'sit' and write
    if i == 4: sys.stdout.write('\n')  # write a blank line after the 5th line

Rewriting a file in place is often done by saving the old copy with a modified name. Unix folks add a ~ to mark the old one. Windows folks do all kinds of things -- add .bak or .old -- or rename the file entirely or put the ~ on the front of the name.

import shutil
shutil.move( afile, afile+"~" )

destination= open( aFile, "w" )
source= open( aFile+"~", "r" )
for line in source:
    destination.write( line )
    if <some condition>:
        destination.write( >some additional line> + "\n" )
source.close()
destination.close()

Instead of shutil, you can use the following.

import os
os.rename( aFile, aFile+"~" )