KeyError字典转换Python

Question

I would like to create a program that converts money from one type of currency to another. So far this is my code:

def read_exchange_rates(exchange_file_name):
    #reads file with exchange rates formatted like USD,1. Each line is a 3 letter currency code and a float to convert it to USD. 
    f=open(exchange_file_name,"r")
    answer={}
    for line in f:
        k, v = line.split(",")
        answer[k] = float(v)
    return answer
    f.close()
    pass
class Money:

    exchange_rates = read_exchange_rates(rate_file)
    #calls previously defined function to read file with exchange rates
    def __init__ (self, monamount, code):
        self.monamount=monamount
        self.code=code
    def to(self, othercode):
        i = self.monamount/self.exchange_rates[self.code]
        j = i*self.exchange_rates[self.othercode]
        return othercode+str(j)

It should return the converted amount along with it's currency code (othercode) but instead it returns a KeyError. If I type

a=Money(650,'USD')
b=a.to('GBP')

it should return GBP somenumber. This is the error. Thank you!

Traceback (most recent call last):
  File "<pyshell#126>", line 1, in <module>
    b=a.to('GBP')
  File "<pyshell#124>", line 9, in to
    i = self.monamount/self.exchange_rates[self.code]
KeyError: 'USD'

Answer 1

Are you sure your file contains 'USD' key?

Here is a slightly modified and simplified code (so I didn't have to create an exchange file):

def read_exchange_rates():
    answer={}
    answer['USD'] = 1
    answer['GBP'] = 0.76
    return answer

class Money:
    exchange_rates = read_exchange_rates()
    def __init__ (self, monamount, code):
        self.monamount = monamount
        self.code = code

    def to(self, othercode):
        i = self.monamount/self.exchange_rates[self.code]
        j = i*self.exchange_rates[othercode]
        return othercode + str(j)

a = Money(650,'USD')
b = a.to('GBP')
print(b)

This prints out GBP494.0 .