StringIO vs array.tofile() ?

[jaf]

Hi all,

I'm seeing some behaviour in Python that I didn't expect.  Is it a bug
in the language, or am I doing something wrong?
Here is an example program to demonstrate the problem:

-----------------------------------------------------------------
import array
import StringIO

a  = array.array('l', [1])
io = StringIO.StringIO()
a.tofile(io)
print io.getvalue()  # we never get here
-----------------------------------------------------------------

I would expect this program to print out the binary representation of
the array (a) and exit.
Instead, I get this TypeError exception:

$ python test.py
Traceback (most recent call last):
  File "test.py", line 9, in ?
    a.tofile(io)
TypeError: arg must be open file

Why is that?  Isn't a StringIO a valid "open file"?

Thanks for any advice,
Jeremy

ps I tried with a cStringIO too, it made no difference