Subclass of web.py's GroupedDropdown that allows mixing option groups with plain options

my_grouped_dropdown.py

import web

class MyGroupedDropdown (web.form.GroupedDropdown):

    """Subclass of GroupedDropdown that allows mixing option groups
    with plain options, i.e. accepts tuples of strings as well as
    tuples of tuples of strings.

    >>> MyGroupedDropdown(name='example', args=(('Swedish Cars',
    (('v', 'Volvo'), ('s', 'Saab'))), ('German Cars', (('m', 'Mercedes'),
    ('a', 'Audi'))), ('d', 'Delorean')), value='a').render()

    u'<select id="example" name="example">\n <optgroup label="Swedish
    Cars">\n <option value="v">Volvo</option>\n <option
    value="s">Saab</option>\n </optgroup>\n <optgroup label="German
    Cars">\n <option value="m">Mercedes</option>\n <option
    selected="selected" value="a">Audi</option>\n </optgroup>\n
    <option value="d">Delorean</option>\n</select>\n'
    """

    def render(self):
        attrs = self.attrs.copy()
        attrs['name'] = self.name
        
        x = '<select %s>\n' % attrs
        
        for arg in self.args:
            if (isinstance(arg, (list, tuple)) and
                isinstance(arg[1], (list, tuple))):
                label, options = arg
                x += '  <optgroup label="%s">\n' % web.net.websafe(label)
                for opt in options:
                    x += self._render_option(opt, indent = '    ')
                x +=  '  </optgroup>\n'
            else:
                x += self._render_option(arg, indent = '  ')

        x += '</select>\n'
        return x