How to use the parsel.csstranslator.XPathExpr function in parsel

def xpath_attr_functional_pseudo_element(self, xpath, function):
        """Support selecting attribute values using ::attr() pseudo-element
        """
        if function.argument_types() not in (['STRING'], ['IDENT']):
            raise ExpressionError(
                "Expected a single string or ident for ::attr(), got %r"
                % function.arguments)
        return XPathExpr.from_xpath(xpath,
                                    attribute=function.arguments[0].value)

def __str__(self):
        path = super(XPathExpr, self).__str__()
        if self.textnode:
            if path == '*':
                path = 'text()'
            elif path.endswith('::*/*'):
                path = path[:-3] + 'text()'
            else:
                path += '/text()'

        if self.attribute is not None:
            if path.endswith('::*/*'):
                path = path[:-2]
            path += '/@%s' % self.attribute

        return path