Actually highlight the block

def highlightBlock(self, text):
        """ Actually highlight the block"""
        # Note that an undefined blockstate is equal to -1, so the first block
        # will have the correct behaviour of starting at 0.
        if self._allow_highlight:
            start = self.previousBlockState() + 1
            end = start + len(text)
            for i, (fmt, letter) in enumerate(self._charlist[start:end]):
                self.setFormat(i, 1, fmt)
            self.setCurrentBlockState(end)
            self.highlight_spaces(text)