file io - seek(), then read(), then write() in python

Question

When running the following python code:

>>> f = open(r"myfile.txt", "a+")        
>>> f.seek(-1,2)                                        
>>> f.read()                                            
'a'                                                     
>>> f.write('
')                                        

I get the following (helpful) exception:

Traceback (most recent call last):      
  File "<stdin>", line 1, in <module>   
IOError: [Errno 0] Error        

The same thing happens when openning with "r+".

Is this supposed to fail? Why?

Edit:

1. Obviously, this is just an example, not what I am actually trying to execute. My actual goal was to verify that the files ends with " ", or add one, before adding the new lines.
2. I am working under Windows XP, and they problem exists in both Python 2.5 and Python 2.6.

3. I managed to bypass the problem by calling seek() again:

   f = open(r"myfile.txt", "a+")
   f.seek(-1,2)
   f.read()
   'a'
   f.seek(-10,2)
   f.write(' ')

The actual parameters of the 2nd seek call don't seem to matter.

Answer 1

This appears to be a Windows-specific problem - see http://bugs.python.org/issue1521491 for a similar issue.

Even better, a workaround given and explained at http://mail.python.org/pipermail/python-bugs-list/2005-August/029886.html, insert:

f.seek(f.tell())

between the read() and write() calls.