为什么熊猫数据框会在“日期”上出现关键错误？

仍然是python的新手。请原谅我问了一个菜鸟方程。使用 mftool 库帮助下载共同基金的资产净值数据。它给出了一个关键的错误'日期'。

将非常感谢识别和帮助解决错误。

输入：

from mftool import Mftool
mf = Mftool()
scheme_codes = mf.get_scheme_codes()
scheme_code_list = [x for x in scheme_codes.keys()]
def HistoricalNav(scheme_code_list, start_date, end_date):
    assert (isinstance(scheme_code_list, list) is True), "Arguement scheme_code_list should be a list" 
  assert (isinstance(start_date, str) is True), "start_date must be a str in %d-%m-%Y format" # checks whether start date is present and is in correct format.
  assert (isinstance(end_date, str) is True), "end_date must be a str in %d-%m-%Y format" # checks whether end date is present and is in correct format

  main_df = pd.DataFrame() #empty dataframe

  for schemes in scheme_code_list:
    data = mf.get_scheme_historical_nav_for_dates(schemes, start_date, end_date) # requesting NAV data from the api.

    df = pd.DataFrame(data['data']) 
    df['scheme_code'] = pd.Series([data['scheme_code'] for x in range(len(df.index))]) #adding Pandas Series(scheme_code) as a column in Pandas Dataframe.
    df['scheme_name'] = pd.Series([data['scheme_name'] for x in range(len(df.index))]) #adding Pandas Series(scheme_name) as a column in Pandas Dataframe.

    df = df.sort_values(by = 'date') # sorting the values of every Scheme code based on Date

    main_df = main_df.append(df) # appending the data in the main_df dataframe.

  main_df = main_df[['scheme_code', 'scheme_name', 'date', 'nav']] #creating names of dataframe columns 
  main_df.reset_index(drop = True, inplace = True) 

  return main_df #Returning the required Dataframe.

values_df = HistoricalNav(scheme_code_list = scheme_code_list[0:5], start_date= '01-05-2021', end_date= '01-05-2021')
values_df

**Error Message**

```Traceback (most recent call last):
  File "C:/Users/am364971/Desktop/Python/Working/amfi.py", line 31, in <module>
    values_df = HistoricalNav(scheme_code_list = scheme_code_list[0:5], start_date= '01-05-2021', end_date= '01-05-2021')
  File "C:/Users/am364971/Desktop/Python/Working/amfi.py", line 22, in HistoricalNav
    df = df.sort_values(by = 'date') # sorting the values of every Scheme code based on Date
  File "C:\Users\am364971\AppData\Local\Programs\Python\Python39\lib\site-packages\pandas\core\frame.py", line 5455, in sort_values
    k = self._get_label_or_level_values(by, axis=axis)
  File "C:\Users\am364971\AppData\Local\Programs\Python\Python39\lib\site-packages\pandas\core\generic.py", line 1684, in _get_label_or_level_values
    raise KeyError(key)
KeyError: 'date'