'Make blank space less apparent by setting the foreground alpha.
This only has an effect when \'Show blank space\' is turned on.
Derived classes could call this function at the end of
highlightBlock().'

def highlight_spaces(self, text, offset=0):

    flags_text = self.document().defaultTextOption().flags()
    show_blanks = (flags_text & QTextOption.ShowTabsAndSpaces)
    if show_blanks:
        format_leading = self.formats.get('leading', None)
        format_trailing = self.formats.get('trailing', None)
        match = self.BLANKPROG.search(text, offset)
        while match:
            (start, end) = match.span()
            start = max([0, (start + offset)])
            end = max([0, (end + offset)])
            if ((end == len(text)) and (format_trailing is not None)):
                self.setFormat(start, end, format_trailing)
            if ((start == 0) and (format_leading is not None)):
                self.setFormat(start, end, format_leading)
            format = self.format(start)
            color_foreground = format.foreground().color()
            alpha_new = (self.BLANK_ALPHA_FACTOR * color_foreground.alphaF())
            color_foreground.setAlphaF(alpha_new)
            self.setFormat(start, (end - start), color_foreground)
            match = self.BLANKPROG.search(text, match.end())
