How to overwrite some bytes in the middle of a file with Python?

Question

I'd like to be able to overwrite some bytes at a given offset in a file using Python.

My attempts have failed miserably and resulted in:

• overwriting the bytes at the offset but also truncating the file just after (file mode = "w" or "w+")
• appending the bytes at the end of the file (file mode = "a" or "a+")

Is it possible to achieve this with Python in a portable way?

Answer 1

Try this:

fh = open("filename.ext", "r+b")
fh.seek(offset)
fh.write(bytes)
fh.close()

Answer 2

According to this python page you can type file.seek to seek to a particualar offset. You can then write whatever you want.

To avoid truncating the file, you can open it with "a+" then seek to the right offset.

Answer 3

Very inefficient, but I don't know any other way right now, that doesn't overwrite the bytes in the middle (as Ben Blanks one does):

a=file('/tmp/test123','r+')
s=a.read()
a.seek(0)
a.write(s[:3]+'xxx'+s[3:])
a.close()

will write 'xxx' at offset 3: 123456789 --> 123xxx456789