Python non-specific write exception

Currently teaching myself Python, and learning file I/O by writing a script to both read from and add text to an existing file. The script runs up until I call the write() method, at which point it throws out a non-specific exception - this is the traceback:

            File "test.py", line 13, in <module>
                f.write(txt)
            IOError: [Errno 0] Error

My code:

            from sys import argv

            script, filename = argv

            f = open(filename, 'a+')

            print("The contents of %s are:") % filename

            print f.read()

            txt = raw_input("What would you like to add? ")

            f.write(txt)

            print("The new contents are:")

            print f.read()

            f.close()

My environment is Python 2.7.3 in Win7, PowerShell, and Notepad++.

What is causing this? How would I fix it? In my understanding, the a+ access mode should allow me to both read and append to the file. Changing the access mode to r+ yields the same exception.

Clarifications:

I have an existing text file (a.txt) with a single word in it that I pass as an argument to the script, like so:
```
        python test.py a.txt
```
I am under an admin account in Windows.

Results:

At the minimum, adding two seek() commands fixes the issue - detailed in the answer post.

Solution

A problem when one tries to add a text of little size: it remains in the buffer, that keeps the text before the real writing is done after receiving more data.
So, to be sure to write really, do as it is described in the doc concerning os.fsync() and flush()

By the way, it is better to use the with statement.

And it's still more better to use binary mode. In your case, there shouldn't be a problem because you just add text after the reading and just use seek(o,o) . But when one wants to move correctly the file's pointer into the bytes of the file, it is absolutely necessary to use binary mode [ the 'b' in open(filename, 'rb+') ]

I personnaly never use 'a+', I've never understood what are its effects.

from sys import argv
from os import fsync

script, filename = argv

with open(filename, 'rb+') as f:
    print("The contents of %s are:") % filename
    print f.read()

    f.seek(0,2)
    txt = raw_input("What would you like to add? ")
    f.write(txt)
    f.flush()
    fsync(f.fileno())

    f.seek(0,0)
    print("The new contents are:")
    print f.read()