'bad file descriptor' is thrown for simple RandomAccessFile.write/seek(0)/read sequence

[ligurio]

>>> f = open("/tmp/charibdushka/tst", "wr+")
>>> import binascii
>>> hs="123456789ABCDEF1"
>>> hb=binascii.a2b_hex(hs)
>>> f.write(hb)
>>> f.seek(0)
>>> f.read()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 9] Bad file descriptor

charybdefs 13