python_Day3：自定义max和min函数

def max_min(l):
    if l == []:
        return (None,None)
    else:
        num_min, num_max = l[0], l[0]
    for i in l:
        if num_min > i:
            num_min = i
        elif num_max < i:
            num_max = i
    return (num_min,num_max)