The expat sax parser in xml.sax.expatreader
does not fully support the InputSource protocol
in xml.sax.xmlreader. It only accepts
byte streams. It ignores the encoding
indicated in the InputStream object and
only uses the encoding read from
the XML or defaults to UTF-8.

Rather than silently doing the wrong thing,
it should raise an error when fed a character stream,
or when given an encoding, via the InputSource
interface.

And most importantly, these limitations should be mentioned
in the documentation.