Writing binary data to middle of a sparse file

Question

I need to compile a binary file in pieces with pieces arriving in random order (yes, its a P2P project)

def write(filename, offset, data) 
    file.open(filename, "ab")
    file.seek(offset) 
    file.write(data) 
    file.close()

Say I have a 32KB write(f, o, d) at offset 1MB into file and then another 32KB write(f, o, d) at offset 0

I end up with a file 65KB in length (i.e. the gap consisting of 0s between 32KB - 1MB is truncated/disappears)

I am aware this may appear an incredibly stupid question, but I cannot seem to figure it out from the file.open(..) modes

Advice gratefully received.

*** UPDATE

My method to write P2P pieces ended up as follows (for those who may glean some value from it)

def writePiece(self, filename, pieceindex, bytes, ipsrc, ipdst, ts): 
    file = open(filename,"r+b")
    if not self.piecemap[ipdst].has_key(pieceindex):
        little = struct.pack('<'+'B'*len(bytes), *bytes) 
        # Seek to offset based on piece index 
        file.seek(pieceindex * self.piecesize)
        file.write(little)
        file.flush()
        self.procLog.info("Wrote (%d) bytes of piece (%d) to %s" % (len(bytes), pieceindex, filename))

    # Remember we have this piece now in case duplicates arrive 
    self.piecemap[ipdst][pieceindex] = True
    file.close()

Note: I also addressed some endian issues using struct.pack which plagued me for a while.

For anyone wondering, the project I am working on is to analyse BT messages captured directly off the wire.

Answer 1

>>> import os
>>> filename = 'tempfile'
>>> def write(filename,data,offset):
...     try:
...         f = open(filename,'r+b')
...     except IOError:
...         f = open(filename,'wb')
...     f.seek(offset)
...     f.write(data)
...     f.close()
...
>>> write(filename,'1' * (1024*32),1024*1024)
>>> write(filename,'1' * (1024*32),0)
>>> os.path.getsize(filename)
1081344

Answer 2

You opened the file in append ("a") mode. All writes are going to the end of the file, irrespective of the calls to seek().