除了ValueErrors

Question

我是使用python进行编程的初学者，并且对异常如何工作感到困惑。我正在为餐馆创建一个菜单列表和字典，如果用户键入无效数字或字符串，我需要它除了值Error以及再次询问用户但仍然得到值错误代码

while True:
    try:
        itemCost = float(raw_input("Please enter the cost of this item (ex: 22.45): "))
        if itemCost < 0: print "Invalid Number, please try again"
    except ValueError:
        print "Invalid Number, please try again"
    except NameError:
        print "Invalid Number, please try again"
    except SyntaxError:
        print "Invalid Number, please try again"

更新

这就是我输入但仍然得到一个值错误代码，因为我使用了一个字符串而不是一个浮点数，但希望它给用户一个错误信息。

Please enter the menu item name (quit to exit): Chicken

Please enter the cost of this item (ex: 22.45): rrr

Traceback (most recent call last):
  File "/Users/Stan/Documents/Intro to Programming/Program Project #2.py", line 14, in <module>
    itemCost = float(raw_input("Please enter the cost of this item (ex: 22.45): "))

ValueError: could not convert string to float: rrr

Process finished with exit code 1