The docs (http://docs.python.org/dev/3.0/library/stdtypes.html#sequence-
types-str-bytes-list-tuple-buffer-range) warn that .fileno and .istty 
should not be implemented by a file like object.

This require client to check if the file object has the attribute before 
they call the method, instead of treating all files the same.

    if hasattr(foo, 'istty'):
        if foo.istty():
            # ...

Instead of:

    if foo.istty():
        # ...

istty can easily return False. fileno can return invalid file descriptor 
number (-1?).