python语句-for

# coding: utf-8
__author__ = 'www.py3study.com'
def cumulative(num):
    sum = 0
    for i in range(1,num + 1):
        sum += i
    return sum   #累加函数，返回累加函数后的值

def main():
    while True:
        print(u"=================================")
        print(u"输入exit退出程序:")
        str_num = input("从1累加到:")
        if str_num == 'exit':
            break
        try:
            sum = cumulative(int(str_num))
        except ValueError:
            print(u"除非退出输入exit,只能输入数字！")
            continue
        print(u"从1累加到{}的总和是{}".format(int(str_num), sum))
if __name__ == '__main__':
    main()