Modular Programming with Python

Programming as a process

Imagine that you are writing a program to calculate the price of overseas purchases. Your company is based in England, and you need to calculate the local price of something purchased in US dollars. Someone else has already written a Python module which downloads the exchange rate, so your program starts out looking something like the following:

def calc_local_price(us_dollar_amount):
    exchange_rate = get_exchange_rate("USD", "EUR")
    local_amount = us_dollar_amount * exchange_rate
    return local_amount

So far so good. Your program is included in your company's online ordering system and the code goes into production. However, two months later, your company starts ordering products not just from the US, but from China, Germany, ...