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

python file seek
Kennedy picture Kennedy · Dec 8, 2010 · Viewed 42.5k times · Source

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

Answer

Marcelo Cantos picture Marcelo Cantos · Dec 8, 2010

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

Related questions

What is the most efficient way to get first and last line of a text file?

I have a text file which contains a time stamp on each line. My goal is to find the time range. All the times are in order so the first line will be the earliest time and the last line …

Read More
Is it safe to open a file several times at once in Python?

I seem to recall cases in lower level languages that opening a file more than once in a program could result in a shared seek pointer. By messing around in Python a bit, this doesn't seem to be happening for …

Read More
How to get the current open file line in python?

Suppose you open a file, and do an seek() somewhere in the file, how do you know the current file line ? (I personally solved with an ad-hoc file class that maps the seek position to the line after scanning the …

Read More