replace string in a file

joep josef.pktd at
Mon Mar 17 17:03:07 CET 2008

On Mar 16, 10:35 pm, sturlamolden <sturlamol... at> wrote:
> On 15 Mar, 21:54, Unknown <cantabile... at> wrote:
> > I was expecting to replace the old value (serial) with the new one
> > (todayVal). Instead, this code *adds* another line below the one found...
> > How can I just replace it?
> A file is a stream of bytes, not a list of lines. You can't just
> replace a line with another, unless they have the exact same length.
> You must rewrite the whole file to get it right.

An example: looks for all 'junk*.txt' files in current directory and
replaces in each line the string 'old' by the string 'new'

import os, glob, fileinput

allfiles = glob.glob(os.getcwd() + '\\junk*.txt') # makes absolute
findstr = 'old'
replstr = 'new'

countlinesfound = 0
for line in fileinput.input(allfiles,inplace=1):
    if line.find(findstr) != -1:
        line = line.replace(findstr,replstr)  # old string , new
        countlinesfound += 1
    print line,    # this writes line back to the file

print countlinesfound

I found something similar in a tutorial when I started to learn
Python, but I don't remember which one.


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