Sorry, left some extraneous code in the last message, here is the right code:

    def _writerthread(self, fhr, fhw, length=None):
        if length is None:
            flag = True
            while flag:
                buf = fhr.read( 512 )
                fhw.write( buf )
                if len( buf ) == 0:
                    flag = False
        else:
            while length > 0:
                buf = fhr.read( min( 512, length ))
                fhw.write( buf )
                length -= len( buf )
        fhw.close()