mmapTest.py

import mmap

with open("/python/trials/mmapTest.txt","wb") as f:
    f.write(b"Hello Python!\n")
    print (f.fileno())

with open("/python/trials/mmapTest.txt","r+b") as f:
    #memory-map the file, size 0 means whole file
    mm=mmap.mmap(f.fileno(), 0)

    #read content via standard file methods
    print(mm.readline()) #prints b"Hello Python!\n"

    #read content via slice notation
    print(mm[:5]) #prints b"Hello"

    #update content using slice notation
    #note that new content must have same size
    mm[6:]=b" world!\n"

    #... and read again using standard file methods
    mm.seek(0)
    print(mm.readline()) #prints b"Hello world!\n"

    #close the map
    mm.close()