python - IF语句错误新变量输入python

Question

这里的问题是我无法让python检查Currency1是否在字符串中，如果不是，则打印存在错误，但如果Currency1在字符串中，则继续并要求用户输入Currency2，然后检查再说一遍。

Answer 1

您实际上是在尝试：

if type(Currency1) in (float, int):
   ...

但isinstance这里更好：

if isinstance(Currency1,(float,int)):
   ...

甚至更好，您可以使用numbers.Number抽象基类：

import numbers
if isinstance(Currency1,numbers.Number):

---

虽然 ...Currency1 = str(raw_input(...))会保证这Currency1是一个字符串（不是整数或浮点数）。实际上，raw_input做出这样的保证，str这里的额外内容只是多余的:-)。

如果您想要一个函数来检查字符串是否可以转换为数字，那么我认为最简单的方法就是尝试一下，看看：

def is_float_or_int(s):
    try:
        float(s)
        return True
    except ValueError:
        return False

Answer 2

您可以使用try-except：

def get_currency(msg):
    curr = input(msg)
    try:
        float(curr)
        print('You must enter text. Numerical values are not accepted at this stage')
        return get_currency(msg)  #ask for input again
    except:
        return curr               #valid input, return the currency name

curr1=get_currency('Please enter the currency you would like to convert:')
curr2=get_currency('Please enter the currency you would like to convert into:')
ExRate = float(input('Please enter the exchange rate in the order of, 1 '+curr1+' = '+curr2)) 
Amount = float(input('Please enter the amount you would like to convert:'))
print (Amount*ExRate)

输出：

$ python3 foo.py

Please enter the currency you would like to convert:123
You must enter text. Numerical values are not accepted at this stage
Please enter the currency you would like to convert:rupee
Please enter the currency you would like to convert into:100
You must enter text. Numerical values are not accepted at this stage
Please enter the currency you would like to convert into:dollar
Please enter the exchange rate in the order of, 1 rupee = dollar 50
Please enter the amount you would like to convert: 10
500.0