'Generate an available name for a field'

def _get_free_field_name(self, field):

    num = 0
    field_name = ((unicode(field.name) + u'___') + unicode(num))
    while (field_name in self.fields):
        num += 1
        field_name = ((unicode(field.name) + u'___') + unicode(num))
    return field_name
