Currently gst.Pad.query_* methods return None when the query fails.
This is particularly annoying as when a query succeeds the result is a two element tuple, so you it's natural to unpack it like

a, b = pad.query_*(...)

except that when there's a failure, python throws an exception at you as None can't be unpacked in two items.
The methods should raise exceptions or at least return (None, None).