How to seek and append to a binary file in python?

问题:

I am having problems appending data to a binary file. When i seek() to a location, then write() at that location and then read the whole file, i find that the data was not written at the location that i wanted. Instead, i find it right after every other data/text.

My code

file = open('myfile.dat', 'wb')
file.write('This is a sample')
file.close()

file = open('myfile.dat', 'ab')
file.seek(5)
file.write(' text')
file.close()

file = open('myfile.dat', 'rb')
print file.read()  # -> This is a sample text

You can see that the seek does not work. How do i resolve this? are there other ways of achieving this?

Thanks

回答1:

On some systems, 'ab' forces all writes to happen at the end of the file. You probably want 'r+b'.

回答2:

r+b should work as you wish

回答3:

Leave out the seek command. You already opened the file for append with 'a'.

回答4:

NOTE : Remember new bytes over write previous bytes

As per python 3 syntax

with open('myfile.dat', 'wb') as file:
    b = bytearray(b'This is a sample')
    file.write(b)

with open('myfile.dat', 'rb+') as file:
    file.seek(5)
    b1 = bytearray(b'  text')
    #remember new bytes over write previous bytes
    file.write(b1)

with open('myfile.dat', 'rb') as file:
    print(file.read())

OUTPUT

b'This   textample'

remember new bytes over write previous bytes