StringIO.readline() returns ''

[Max]

I'm using StringIO for the first time (to buffer messages recieved from 
a socket). I thought it would be a simple matter of writing the stuff to
the buffer and then calling readline, but that doesn't seem to work:

 >>> buf = StringIO.StringIO()
 >>> buf.write("Foo\n")
 >>> buf.write("Bar\n")
 >>> buf.flush()
 >>> buf.readline()
''

I expected buf.readline() to return 'Foo\n'. What am I doing wrong?

--Max