Writing Back Into The Same File After Reading From The File

My aim is to read line from the file , strip the blank spaces at the end of it and write back into the same file. I have tried the following code: with open(filename, 'r+') as f:

Solution 1:

Use a temporary file. Python provides facilities for creating temporary files in a secure manner. Call example below with: python modify.py target_filename

import tempfile
 import sys

 defmodify_file(filename):

      #Create temporary file read/write
      t = tempfile.NamedTemporaryFile(mode="r+")

      #Open input file read-only
      i = open(filename, 'r')

      #Copy input file to temporary file, modifying as we gofor line in i:
           t.write(line.rstrip()+"\n")

      i.close() #Close input file

      t.seek(0) #Rewind temporary file to beginning

      o = open(filename, "w")  #Reopen input file writable#Overwriting original file with temporary file contents          for line in t:
           o.write(line)  

      t.close() #Close temporary file, will cause it to be deletedif __name__ == "__main__":
      modify_file(sys.argv[1])

References here: http://docs.python.org/2/library/tempfile.html

Solution 2:

The problem with your approach is that you need both an input stream and an output stream, which can point at different places in the same file. If you want to use f.seek() then you will need to store the position using f.tell() after each read and write. For example:

f = open(filename, 'r+')
while True:
    i = f.readline()
    if i == '': breakin = f.tell()
    f.seek(out)
    f.write(i.rstrip()+"\n")
    out = f.tell()
    f.seek(in)

But that's confusing and prone to errors. If the file isn't too big, why not read it all into memory and then write it back out again?

in = open(filename, 'r')
lines = in.read()
in.close()
out = open(filename, 'w')
out.write([line.rstrip()+'\n'for line inlines.split('\n')])
out.close()

If the file is too large to fit into memory, then write the lines to a temporary file and then rename the file when you are done:

out = open(filename+'.tmp', 'w')
with open(filename, 'r') as f:
    for i in f:
        out.write(i.rstrip()+"\n")
out.close()
os.rename(filename+'.tmp', filename)