请使用迭代查找一个list中最小和最大值，并返回一个tuple：

# -*- coding: utf-8 -*-
def findMinAndMax(L):
    minNum = maxNum = 0
    if len(L) == 0:
        minNum = maxNum = None
    else:
        minNum = maxNum = L[0]
        for l in L:
            if l < minNum:
                minNum = l
            if l > maxNum:
                maxNum = l       
    return (minNum, maxNum)

# 测试
if findMinAndMax([]) != (None, None):
    print('测试失败!')
elif findMinAndMax([7]) != (7, 7):
    print('测试失败!')
elif findMinAndMax([7, 1]) != (1, 7):
    print('测试失败!')
elif findMinAndMax([7, 1, 3, 9, 5]) != (1, 9):
    print('测试失败!')
else:
    print('测试成功!')