To find out what really happens, use pdb to step through the example.
This gives much better insight than adding a printf() call to
PyObject_Call(). I've notice myself when stepping through I/O that there
are a lot of checks for closed -- this may have to do with the stacking
text -> buffer -> binary though. I've also noticed overridden isinstance
checks in abc.py being called, which surprised me slightly. I haven't
had time to look into this further though.