code: def test_download(ticker_list):
      df = YahooDownloader(
          start_date="2019-01-01",
          end_date="2019-02-01",
          ticker_list=ticker_list,
      ).fetch_data()
      assert isinstance(df, pd.DataFrame)
  
imports: from __future__ import annotations
  import pandas
  import pytest
  from finrl.meta.preprocessor.yahoodownloader import YahooDownloader
label_desc: Use image node_id 2 for calling a global function with example usage: test_download(ticker_list) without return types