Reading a file and then writing something back

[Kevin T. Ryan]

Hi All - 

I'm not sure, but I'm wondering if this is a bug, or maybe (more
likely) I'm misunderstanding something...see below:

>>> f = open('testfile', 'w')
>>> f.write('kevin\n')
>>> f.write('dan\n')
>>> f.write('pat\n')
>>> f.close()
>>> f = open('testfile', 'r+')
>>> f.readline()
'kevin\n'
>>> f.readline()
'dan\n'
>>> f.write('chris\n')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
IOError: (0, 'Error')

I've figured out that I can do an open('testfile', 'r+') and then seek
and write something (without an error), but it just seems odd that I
would get an IOError for what I was trying to do.  Oh, and I also
tried to do "f.flush()"  before the write operation with no luck. 
I've searched google, but can't seem to find much.  Any thoughts??? 
TIA,

Kevin