def STD_DEV_POP(*expression):
    """
    Calculates the population standard deviation of the input values.
    Use if the values encompass the entire population of data you want to represent
    and do not wish to generalize about a larger population.
    See https://docs.mongodb.com/manual/reference/operator/aggregation/stdDevPop/
    for more details
    :param expression: expression/expressions
    :return: Aggregation operator
    """
    return {'$stdDevPop': expression} if len(expression) > 1 else {'$stdDevPop'
        : expression[0]} if expression else {}
