It looks like a bug, because __getslice__ is deprecated since 2.0.

If you subclass a builtin type and override the __getitem__ method, you need to override the (deprecated) __getslice__ method too.
And if you run your program with "python -3", it 

Example script:


class Upper(unicode):

    def __getitem__(self, index):
        return unicode.__getitem__(self, index).upper()

    #def __getslice__(self, i, j):
        #return self[i:j:]


if __name__ == '__main__':
    text = Upper('Lorem ipsum')

    print text[:]
    print text[::]

This is because unicode implements __getslice__.

OK, but it yields Python 3 DeprecationWarning in the subclass.
And there's no workaround to get rid of the deprecation. 

If it is the correct behaviour, maybe some words could be added about subclassing builtin types:
http://docs.python.org/reference/datamodel.html#additional-methods-for-emulation-of-sequence-types

OK, I said nothing, it is already in the doc.

:-)