温度转换异常处理

try:
    a=input()
    if a [-1] in ['c','C']:
        f=1.8*eval(a[0:-1])+32
        print("{:.2f}F".format(f))
    elif a [-1] in ['f','F']:
        c=(eval(a[0:-1])-32)/1.8
        print("{:.2f}C".format(c))
    else:
        print("输入错误，末位只能是'C','c','F','f'")
except NameError :
    print("试图访问的变量名不存在")
except SyntaxError:
    print('存在语法错误')