reading_writing.py

 # Reading

import mmap
with open("hello.txt",'r') as f:
    with mmap.mmap(f.fileno(),0,access=mmap.ACCESS_READ) as m:
        print("First 10 bytes via read  -->",m.read(10))
        print("First 10 bytes via slice  -->",m[:10])
        print("2nd 10 bytes via read  -->",m.read(10))       
        
"""
First 10 bytes via read  --> b'Lorem ipsu'
First 10 bytes via slice  --> b'Lorem ipsu'
2nd 10 bytes via read  --> b'm dolor si'
"""

# writing 
import mmap
import shutil
shutil.copyfile("hello.txt","hello_copy.txt") # copying the file 
word = b"python"
rev_word = word[::-1]



with open("hello_copy.txt","r+") as f:
    with mmap.mmap(f.fileno(),0,access = mmap.ACCESS_WRITE) as m:
        print("Before reversing the string ---> {}".format(m.readline().strip()))
        m.seek(0) # coming to initial position

        # the mmap.mmap.find() function returns the lowest 
        # index in the object where the subsequence sub is found.
        loc = m.find(word)
        m[loc:loc + len(word)] = rev_word
        # even though we just wrote into The mmap it may not occure in the mmap
        # until the file is closed(mmap.close) hence we use flush which forces the buffer 
        # content to the memory map(mmap)
        m.flush()

        m.seek(0)

        print("After reversing the string ---> {}".format(m.readline().strip()))

        f.seek(0)
        print("After reversing the string ---> {}".format(f.readline().strip()))
        # IN This case we observe how ACCESS_WRITE effects the memory(RAM) as well as 
        # the file(Disk)

"""
Before reversing the string ---> b'Lorem ipsum dolor sit amet, consectetuer python adipiscing elit.'
After reversing the string ---> b'Lorem ipsum dolor sit amet, consectetuer nohtyp adipiscing elit.'
After reversing the string ---> Lorem ipsum dolor sit amet, consectetuer nohtyp adipiscing elit.
"""  

# copying
import mmap
import shutil
shutil.copyfile("hello.txt","hello_copy.txt") # copying the file 
word = b"python"
rev_word = word[::-1]



with open("hello_copy.txt","r+") as f:
    with mmap.mmap(f.fileno(),0,access = mmap.ACCESS_COPY) as m:
        print("Before reversing the string ---> {}".format(m.readline().strip()))
        m.seek(0) # coming to initial position

        # the mmap.mmap.find() function returns the lowest 
        # index in the object where the subsequence sub is found.
        loc = m.find(word)
        m[loc:loc + len(word)] = rev_word
        # even though we just wrote into The mmap it may not occure in the mmap
        # until the file is closed(mmap.close) hence we use flush which forces the buffer 
        # content to the memory map(mmap)
        m.flush()

        m.seek(0)

        print("After reversing the string ---> {}".format(m.readline().strip()))

        f.seek(0)
        print("After reversing the string ---> {}".format(f.readline().strip()))
        # in this case only the memory is affected not the file
        """
        Before reversing the string ---> b'Lorem ipsum dolor sit amet, consectetuer python adipiscing elit.'
        After reversing the string ---> b'Lorem ipsum dolor sit amet, consectetuer nohtyp adipiscing elit.'
        After reversing the string ---> Lorem ipsum dolor sit amet, consectetuer python adipiscing elit.
        """