如何将列表的内容传递到现有文件中？

Question

exchangerates = []
newrate = float(input("Enter amount:"))
with open("ExchangeRates.txt","r") as readfile:
    for line in readfile:
        exchangerates.append(line.strip().split(","))
        for line in exchangerates:
            if line[0] == currency1 and line[1] == currency2:
                line[2] = newrate
                print("The exchange rate for",currency1,"and",currency2,"has been changed to",newrate)
        next
        return 0

该代码有效，但是我需要知道如何将上面创建的列表中的信息传递到文件中，显示为“ Exchangerates.txt”。 我该怎么做？

样本输入和输出：

输入货币1：日元

输入货币2：美元

输入数量：3

日元和美元的汇率已更改为3.0

这就是我的代码中发生的情况，这是预期的。 但是，它不会写入文件，并且文件中的信息也不会更改，这是我的问题。

Answer 1

一种特别简单的方法是使用Pickle ，我认为您正在尝试执行以下操作：

import pickle
try:
    with open("ExchangeRates.txt", "rb") as readfile:
        exchangerates = pickle.load(readfile)
except FileNotFoundError:
    exchangerates = {}

currency1 = input("Enter currency 1: ")
currency2 = input("Enter currency 2: ")
newrate = input("Enter exchange rate: ")

if currency1 in exchangerates:
    if currency2 not in exchangerates[currency1] or exchangerates[currency1][currency2] != newrate:
        exchangerates[currency1][currency2] = newrate
        print("The exchange rate for",currency1,"and",currency2,"has been changed to",newrate)
else:
    exchangerates[currency1] = {currency2: newrate}
    print("The exchange rate for",currency1,"and",currency2,"has been changed to",newrate)

with open("ExchangeRates.txt", "wb") as writefile:
    pickle.dump(exchangerates, writefile)

请注意，这会将汇率存储为字典，例如： {'USD': {'GBP': 1.6, 'YEN': 3}} 。 如果需要的话，让它添加/检查并更新逆汇率也很简单。

Answer 2

您可以使用更简单的方法来做到这一点：

f = open("ExchangeRates.txt","r")
exchangerates = f.read()
f.close()


lines = exchangerates.split("\n")

# do the stuff you want to the list with the loops in list of lines

exchangerates = "\n".join(lines)

f = open("ExchangeRates.txt","w")
f.write(exchangerates)
f.close()