import mmap

f = open('bar', 'w+')
f.write("abcdefghijklmnopqrstuvwxyz")
f.close()

f = open('bar', 'r+')
f.seek(0, 2)
size = f.tell()

f.write("\x00"*10)
f.flush()

m = mmap.mmap(f.fileno(), size+10)
m.move(5, 0, 26)