Pythonic way to overwrite a file

Jean-Michel Pichavant jeanmichel at
Wed Jun 17 20:09:04 CEST 2009

Cameron Pulsford wrote:
> Hey all, hopefully a simple question.
> I'm writing a simple python tool that opens a file, and does something 
> like
> for line in file.readlines():
>     temp.write(line.doStuff())
> However, I want to provide the option do this "in place", as in have 
> the destination file be the same as the source file. Currently, I am 
> writing to a temp file and then using "os.system('mv %s %s' % 
> (dstfile, srcfile))" to copy the destination file onto the soruce 
> file. This is extremely ugly though, and will only work on unix based 
> systems (I'm guessing, unless windows has mv too). Is there a more 
> pythonic way to do this? Ideally I'd like to change the file as I go 
> through it and not deal with a second file at all. That wouldn't have 
> any atomicity though... What would be the most pythonic+safest way to 
> do this? 
> Thanks in advance

Altering directly the file is dangerous, what if something goes wrong 
during the process ?
Create a temp file and copying it if successful is your best bet.

I guess using python modules like tempfile and shutil are  a pythonic 
way to do it :

import tempfile
import shutil

In [14]: tempfile.NamedTemporaryFile?
Definition:     tempfile.NamedTemporaryFile(mode='w+b', bufsize=-1, 
suffix='', prefix='tmp', dir=None)
    Create and return a temporary file.
    'prefix', 'suffix', 'dir' -- as for mkstemp.
    'mode' -- the mode argument to os.fdopen (default "w+b").
    'bufsize' -- the buffer size argument to os.fdopen (default -1).
    The file is created as mkstemp() would do it.

    Returns an object with a file-like interface; the name of the file
    is accessible as  The file will be automatically deleted
    when it is closed.

In [7]: shutil.copy?
Definition:     shutil.copy(src, dst)
    Copy data and mode bits ("cp src dst").

    The destination may be a directory.

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