python核心编程 5-6题 简单计算器程序！(python课后题)

i=0
while i==0:
    a = raw_input('>>>')

    try:
        if a[1] == '+':
            print float(a[0])+float(a[2])
        elif a[1] == '-':
            print float(a[0])-float(a[2])
        elif a[2] == '*':
            print float(a[0])**float(a[3])
        elif a[1] == '/':
            print float(a[0])/float(a[2])
        elif a[1] =='*':
            print float(a[0])*float(a[2])
        elif a[1] == '%':
            print float(a[0])%float(a[2])
        else:
            print "Sorry: I don't know!"
    except:
        print "Please input a correct formula!"