我发现自己无法解决这个问题(自动完成税务文书工作).我有两个数据框:例如，一个具有欧元/美元汇率的季度历史记录，另一个具有我自己的发票，例如:

I find myself lost trying to solve this problem (automating tax paperwork). I have two dataframes: one with the quarterly historical records of EUR/USD exchange rates, and another with my own invoices, as an example:

导致:

我需要在sales['Rate']栏中填充来自rates['Rate']的正确汇率，即:

I would need to have the sales['Rate'] column filled with the proper exchange rates from the rates['Rate'], that is to say:

• 如果sales['Currency']是'EUR'，请不要理会.
• 对于sales的每一行，在rates中找到具有匹配的'Date'的行；抓取非常rates['Rate']的值并将其放入sales['Rate']
• 奖励:如果没有匹配的'Date'(例如，在假期期间，交易所市场关闭)，请检查上一行直到找到合适的值.

• if sales['Currency'] is 'EUR', leave it alone.
• for each row of sales, find the row in rates with matching 'Date'; grab that very rates['Rate'] value and put it in sales['Rate']
• bonus: if there's no matching 'Date' (e.g. during holidays, the exchange market is closed), check the previous row until a suitable value is found.

完整结果应如下所示(请注意，第2行的费率自2017年7月7日开始):

The full result should look like the following (note that row #2 has the rate from 2017-07-07):

我尝试遵循其他问题的一些建议解决方案，但没有运气. 提前非常感谢您

I've tried to follow several suggested solutions from other questions, but with no luck. Thank you very much in advance

我使用pd.merge_asof

然后