Cross-platform issues with file's a+ mode

[Michael George Lerner]

I was helping someone debug his code today .. it worked fine on Linux, but
gave a strange error message on Windows.  Here's the snippet that goes
wrong in Python 2.2 under Windows (it also happens under 2.3a1 under
Windows and Enthought's 2.3.3, so I think it's pretty generic) .. :

>>> os.remove('test.txt') 
>>> f = file('test.txt','a+') 
>>> f.readline()
'' 
>>> f.write('beep') 
Traceback (most recent call last):
  File "<pyshell#48>", line 1, in ?
    f.write('beep') 
IOError: (0, 'Error')

but .. I can make things work by seeking:

>>> f.seek(0) 
>>> f.write('beep')
>>>

I don't know whether it's important or not, but if I do an f.tell() after
the readline(), it says 0L, which seems reasonable.

For comparison (and this is also true under, e.g., Linux 2.3a1),

bash-2.04$ python 
Python 2.2 (#1, Feb 18 2002, 14:48:51) [C] on sunos5
Type "help", "copyright", "credits" or "license" for more information. 
>>> import os 
>>> os.remove('test.txt') 
>>> f = file('test.txt','a+') 
>>> f.readline() 
'' 
>>> f.write('beep') 
>>>

What's going on here?  Should I expect this behavior?  I've never needed
to use a+ mode before .. is it just known to be funny or have interesting
cross-platform issues?

Thanks,

-michael