'Get a list of attribute values for the items in this group
@param key: the attribute key
@return: the list of values'

def get_values(self, key):

    if (self.items is None):
        return None
    values = []
    append = values.append
    extend = values.extend
    for item in self.items:
        raw = item.get('_row')
        if (raw is None):
            value = item.get(key)
        else:
            try:
                value = raw.get(key)
            except (AttributeError, TypeError):
                value = item.get(key)
        if (type(value) is list):
            extend(value)
        else:
            append(value)
    return values
