python - 赋值前引用的局部变量 'df'

Question

我不知道如何处理这个练习。

“您可以使用此模板获取 DJIA 成员调整后的收盘价。

首先，您应该在线下载 DJIA 成员列表。然后读入成员，在main函数中写一个for循环，获取需要的数据。

提示，在 get_EOD_data 函数中使用‘tickers’参数”

import requests
import pandas as pd
from pandas.compat import StringIO
import numpy as np
import datetime as dt
from dateutil.relativedelta import relativedelta

def get_EOD_data(api_token='5cb671b0b4a790.35526238', session = None, tickers = 'AXP', start_date = dt.datetime(2018,1,1), end_date = dt.datetime(2018,12,31)):
    symbols = tickers
    if session is None:
        session = requests.Session()


    url = 'https://eodhistoricaldata.com/api/eod/%s.US'
    params = {"api_token": api_token, "from": start_date, "to": end_date}
    r = session.get(url, params = params)
    if r.status_code == requests.codes.ok:

        # Use the parameters index_col and usecols to control which columns you want to scrape
        df = pd.read_csv(StringIO(r.text), skipfooter = 1, parse_dates = [0], engine = 'python', na_values=['nan'])

    df.fillna(method = 'ffill', inplace = True)
    df.fillna(method = 'bfill', inplace = True)
    return df

def main():
    # Add parameters of the function get_EOD_data to control the data you want to scrape
    df_data = get_EOD_data()
    print(df_data)

if __name__ == '__main__':
    main()

Traceback (most recent call last):
  File "/Users/katewang/Desktop/Test/scrape_data.py", line 32, in <module>
    main()
  File "/Users/katewang/Desktop/Test/scrape_data.py", line 28, in main
    df_data = get_EOD_data()
  File "/Users/katewang/Desktop/Test/scrape_data.py", line 22, in get_EOD_data
    df.fillna(method = 'ffill', inplace = True)
UnboundLocalError: local variable 'df' referenced before assignment

Answer 1

在此部分中

. . . . 

if r.status_code == requests.codes.ok:

    # Use the parameters index_col and usecols to control which columns you want to scrape
    df = pd.read_csv(StringIO(r.text), skipfooter = 1, parse_dates = [0], engine = 'python', na_values=['nan'])

df.fillna(method = 'ffill', inplace = True)
. . . 

你会注意到变量 df 的赋值是在一个条件语句中，它可能评估 false 因为 r.status_code 和 requests.codes.ok 不等价