How to replace the line in which I'm currently?

Question

I have a python code like this:

with open('myFile') as f:
        next(f) # skip first line
        for line in f:
            items = line.split(';')
            if len(items) < 2:
                # now I want to replace the line with s.th. that i write

how to replace the line with s.th. that I want to write?

Generally when you want to re-write a file in place, it's much easier to write a new file and them move the new file on top of the old one. — mgilson, Apr 25 '13 at 13:47
The way given by @mgilson is easier to implement. Just keep writing the lines you want to the other file and replace the old file with the new file. — Sukrit Kalra, Apr 25 '13 at 13:48
definitely not easy. It's a lot easier if the lines are the same length, but if they're not, there really is no good way to do it. — mgilson, Apr 25 '13 at 13:49
okay, so I will take the other way ant write a new file and replace it then. Can you show me the code for deleting and changing the name of a file? — gurehbgui, Apr 25 '13 at 13:50
This may be helpful - http://stackoverflow.com/questions/39086/search-and-replace-a-line-in-a-file-in-python?rq=1 — Sukrit Kalra, Apr 25 '13 at 13:51
@gurehbgui use the `os` module for renaming and deleting a file. — Ashwini Chaudhary, Apr 25 '13 at 13:52

iruvar · Answer 1 · 2013-04-25T14:03:28.057

6

Use the fileinput module's inplace functionality. Refer Optional in-place filtering section at fileinput. Something like this:

import fileinput
import sys
import os

for line_number, line in enumerate(fileinput.input('myFile', inplace=1)):
  if line_number == 0:
    continue
  items = line.split(';')
  if len(items) < 2:
    sys.stdout.write('blah' + os.linesep)
  else:
    sys.stdout.write(line)

edited Apr 25 '13 at 14:03

answered Apr 25 '13 at 13:52

iruvar

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are you sure `sys.stdout.write` will really write into the file ? ? I am not. – Stephane Rolland Apr 25 '13 at 13:56
1

@StephaneRolland, yes it will. See the optional in-place filtering section under [fileinput](http://docs.python.org/2/library/fileinput.html) – iruvar Apr 25 '13 at 13:59
Maybe add a note about it in the answer. – Stephane Rolland Apr 25 '13 at 14:01
I'm getting the error, what WindowsError: [Error 32] the processcan not access the file, because it is used by a other process. But I dont have the file opend with any other tool etc – gurehbgui Apr 25 '13 at 14:19
@gurehbgui, the documentation states that the inplace implementation does not work on MS-DOS 8+3 systems...is your file on a FAT filesystem? – iruvar Apr 25 '13 at 14:27
ah okay, so I will use the other solution. No its on ntfs – gurehbgui Apr 25 '13 at 16:04

Ashwini Chaudhary · Accepted Answer · 2013-04-25T14:04:50.613

Open file in r+ mode, read it's content in a list first and then after truncation you can write the new data back to the file.

'r+' opens the file for both reading and writing

If the file is huge then it is better to write this to new file first and then rename.

with open('myFile','r+') as f:
        data=f.readlines()[1:]
        f.truncate(0)          #this will truncate the file
        f.seek(0)             #now file pointer goes to start of the file
        for line in data:     #now write the new data
            items = line.split(';') 
            if len(items) < 2:
                # do something here
            else:
                f.write(line)

I choosed this solution because its also working on ntfs windows systems — gurehbgui, Apr 25 '13 at 16:05

How to replace the line in which I'm currently?

2 Answers2