docs: def download_historical(tickers_list, output_folder):
      """Download historical data from Yahoo Finance.
  
      Downloads full historical data from Yahoo Finance as CSV. The following
      fields are available: Adj Close, Close, High, Low, Open and Volume. Files
      will be saved to output_folder as <ticker>.csv.
  
      :param tickers_list: List of tickers that will be returned.
      :type tickers_list: list of strings
      :param output_folder: Output folder path
      :type output_folder: string
      """
      __validate_list(tickers_list)
      for ticker in tickers_list:
          file_name = os.path.join(output_folder, ticker + '.csv')
          with open(file_name, 'wb') as f:
              base_url = 'http://real-chart.finance.yahoo.com/table.csv?s='
              try:
                  urlopen(base_url + ticker)
                  urlretrieve(base_url + ticker, f.name)
              except:
                  os.remove(file_name)
                  raise RequestError('Unable to process the request. Check if ' +
                                     ticker + ' is a valid stock ticker')