mmap_write_copy.py

#1/usr/bin/env python

# this is from The Python
# Standard Library by example
# ISBN13: 9780321767349

import mmap
import shutil
import contextlib

# Copy the example file
shutil.copyfile('lorem.txt', 'lorem_copy.txt')

word = 'consectetuer'
reversed = word[::-1]

with open('lorem_copy.text', 'r+') as f:
    with contextlib.closing(mmap.mmap(f.fileno(), 0,
                                      access=mmap.ACCESS_COPY)) as m:

        print 'Memory Before:'
        print m.readline().rstrip()
        print 'File Before  :'
        print f.readline().rstrip()
        print

        m.seek(0)  # rewind
        loc = m.find(word)
        m[loc:loc + len(word)] = reversed

        m.seek(0)  # rewind
        print 'Memory After :'
        print m.readline().rstrip()

        f.seek(0)
        print 'File After  :'
        print f.readline().rstrip()