The new io.BufferedRandom implementation in Python 3.1 has a broken seek 
that seems not to properly handle the case when the target of the seek 
lies inside the contents of the file buffer.  It leaves the file object 
in a confused state, such that the next write operation applies after 
the end of the buffer(!) instead of the specified target.

I could reproduce the following symptoms on both Debian Lenny and Mac OS 
X Leopard.  I downloaded the Python 3.1 tarball from python.org, and 
built it by hand using './configure && make'.

$ ./python.exe
Python 3.1 (r31:73572, Aug  3 2009, 02:32:10) 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> open("test", "wb").write(b"A" * 10000)
10000
>>> file = open("test", "rb+")
>>> file.read(10)       # Reads 4096 bytes into file buffer
b'AAAAAAAAAA'
>>> file.tell()
10
>>> file.seek(0)
0
>>> file.tell()
0
>>> file.write(b"B" * 10000)  # This should overwrite the whole file
10000
>>> file.tell()
14096  # Hmm, 0 + 10000 == 14096?
>>> file.close()
>>> d = open("test", "rb").read()
>>> len(d)
14096  # ?!
>>> d[0:10]      # The file should now consist of 10000 Bs...
b'AAAAAAAAAA'
>>> d[4090:4100]
b'AAAAAABBBB'    # ... but the Bs only start after a buffer's worth of 
As.

This bug has actually caused me some subtle, silent data corruption that 
went undetected for quite a while.  Hurray for backups!

The above code works fine in Python 3.0, and its Python 2.5 port also 
produces correct results.

A workaround for 3.1 is to call flush before every seek.