Editing a file 'In Place'

[Simon Foster]

I'm trying to write a simple piece of code to perform some batch edits
on a text file.  So far I have:

import re
import sys

sys.stdout = open( sys.argv[2], 'w' )

pdlpat = r'PDL...:'

pdl = re.compile( pdlpat )

f = open( sys.argv[1], 'r' ).read()
flist = list( f )

startpos = 0
pdlnum = 10

while 1:
    m = pdl.search( f, startpos )
    if not m: break
    flist[m.start():m.end()] = list( 'PDL%03d:' % pdlnum )
    pdlnum += 10
    startpos = m.end() + 1

for i in flist:
    sys.stdout.write( i )


Is there any way to avoid turning the inpuit into a list just so that
I can modify it.  I would like to just do something like this:

import re
import sys

sys.stdout = open( sys.argv[2], 'w' )

pdlpat = r'PDL...:'

pdl = re.compile( pdlpat )

f = open( sys.argv[1], 'r' ).read()

startpos = 0
pdlnum = 10

while 1:
    m = pdl.search( f, startpos )
    if not m: break
    f[m.start():m.end()] = 'PDL%03d:' % pdlnum )
    pdlnum += 10
    startpos = m.end() + 1

print f


But of course, strings are immutable, so this does not work.  Is there
an idiomatic way of doing this?



--
Simon Foster
Cheltenham
England