input函数出现的问题(Python)

参考书<A Learner's Guide to Programming Using the Python Language>,写下如下的程序:

 1 # coding=utf-8
 2 # 以上是为了能在文中添加中文的注释
 3 
 4 def save_transaction(price, credit_card, description):
 5     file = open("transaction.txt", "a")
 6     file.write("%s%07d%16s\n" % (credit_card, price, description))
 7     file.close()
 8     
 9 items = ["DONUT","LATTE","FILTER","MUFFIN"]
10 prices = [1.50, 2.0, 1.80, 1.20]
11 running = True
12 
13 while running:
14     option = 1
15     for choice in items:
16         print(str(option) + "." + choice)
17         option = option + 1
18         
19     print(str(option) + ".Quit")
20     
21     choice = int(input("Choose an option: "))
22     if option == choice:
23         running = False
24     else:
25         #用input的话，如果是以0开始的字符串的话，就会报错
26         credit_card = input("Credit card number: ")
27         while len(credit_card) != 16 :
28             credit_card = input("the length of credit card number must be equal to 16. Please input Credit card number again: ")
29         save_transaction(prices[choice-1]*100, credit_card, items[choice-1])

但是，在运行到第26行的时候，如果输入“0987648900804387”时，会出现如下的错误:

Traceback (most recent call last):
  File "transaction.py", line 26, in <module>
    credit_card = input("Credit card number: ")
  File "<string>", line 1
    0987648900804387
                  ^
SyntaxError: invalid token

上网查了下，注意到一个细节：

input其实是通过raw_input来实现的，原理很简单，就下面一行代码:[参考 http://www.cnblogs.com/linlu11/archive/2009/11/25/1610830.html]

def input(prompt):
    return (eval(raw_input(prompt)))

那么，简单写了测试函数:

>>> eval("0123")
83
>>> eval("0987")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    0987
      ^
SyntaxError: invalid token

>>> eval("0x9d")
157
>>> eval("0x9h")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1
    0x9h
      ^
SyntaxError: unexpected EOF while parsing

可见，在input函数中，调用eval时候，会根据字符串中的第一个或者前2个字符来判断字符串对应是8进制还是16进制的，

如果是0，就是按照8进制来处理：所以上面的输入是 0987648900804387 非法的,不是一个有效的8进制数;

如果是0x，就按照16进制来处理，所以0x9h也是非法的。

因此，原程序中的第26行到28行，可以换成：

1         credit_card = raw_input("Credit card number: ")
2         while len(credit_card) != 16 :
3             credit_card = raw_input("the length of credit card number must be equal to 16. Please input Credit card number again: ")