如何使用 Python 中的 class 变量作为方法的默认参数？

Question

我正在尝试使用 class 变量Stock.today作为方法buy_and_hold的默认参数，但是，我的代码不起作用。 class 变量today将今天的日期作为字符串返回。 如果我手动键入完全相同的字符串（即“2020-6-18”）作为默认参数，则代码有效。 如何使用Stock.today使代码运行？

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import time
from datetime import datetime
import yfinance as yf


class Stock:

    year = time.strftime('%Y')
    month = time.strftime('%m').replace("0","")
    day = time.strftime('%d').replace("0","")
    today = Stock.year + '-' + Stock.month + '-' + Stock.day

    def __init__(self,ticker):

        self.ticker = ticker

    # frequency as string # start and end are strings "Y-M-D" # Benchmark as string
    def buy_and_hold(self,frequency,start,end=Stock.today,yfbenchticker='^SPX'):

        sdate_list =[]
        for date in start.split("-"):
            sdate_list.append(int(date))
        sdate = datetime(year=sdate_list[0],month=sdate_list[1],day=sdate_list[2])

        edate_list = []
        for date in end.split("-"):
            edate_list.append(int(date))
        edate = datetime(year=edate_list[0],month=edate_list[1],day=edate_list[2])

        stock = yf.Ticker(self.ticker).history(interval=frequency,start=sdate,end=edate)
        stock.drop(['Dividends','Stock Splits'],axis=1, inplace=True)

        bench = yf.Ticker(yfbenchticker).history(interval=frequency,start=sdate,end=edate)
        bench.drop(['Dividends','Stock Splits'],axis=1, inplace=True)

        # Interpolate missing values
        if stock.isnull().values.any() == True:
            stock['Close'].fillna(value=stock['Close'].mean())

        if bench.isnull().values.any() == True:
            bench['Close'].fillna(value=stock['Close'].mean())

        # Calculate cumulative returns
        stock['Cumulative return'] = stock['Close'] / stock['Close'].iloc[0]
        bench['Cumulative return'] = bench['Close'] / bench['Close'].iloc[0]


        # Plot cumulative returns
        stock['Cumulative return'].plot(color='#00008b',figsize=(16,8))
        plt.plot(bench['Cumulative return'], color='#cd2b00')
        plt.xlabel('Date')
        plt.ylabel('Multiplier')
        plt.legend((self.ticker,yfbenchticker),loc=0,prop={'size':10})

        plt.show()

        return bench


amd = Stock("AMD")

Answer 1

解决此问题的一种方法是使用默认值 None，然后在 function 正文中分配您实际需要的值：

def buy_and_hold(self,frequency,start,end=None,yfbenchticker='^SPX'):
    if end is None:
        end = Stock.today

但是，正如 Mark Ransom 评论的那样，您应该只能使用end=today 。

Answer 2

使用 self 而不是 Stock 并将初始化放在构造函数中