code: def default_returns_func(symbol, start=None, end=None):
      
  
      if start is None:
          start = '1/1/1970'
      if end is None:
          end = _1_bday_ago()
  
      start = get_utc_timestamp(start)
      end = get_utc_timestamp(end)
  
      if symbol == 'SPY':
          filepath = data_path('spy.csv')
          rets = get_returns_cached(filepath,
                                    get_symbol_returns_from_yahoo,
                                    end,
                                    symbol='SPY',
                                    start='1/1/1970',
                                    end=datetime.now())
          rets = rets[start:end]
      else:
          rets = get_symbol_returns_from_yahoo(symbol, start=start, end=end)
  
      return rets[symbol]
docs: Gets returns for a symbol.
      Queries Yahoo Finance. Attempts to cache SPY.
  
      Parameters
      ----------
      symbol : str
          Ticker symbol, e.g. APPL.
      start : date, optional
          Earliest date to fetch data for.
          Defaults to earliest date available.
      end : date, optional
          Latest date to fetch data for.
          Defaults to latest date available.
  
      Returns
      -------
      pd.Series
          Daily returns for the symbol.
           - See full explanation in tears.create_full_tear_sheet (returns).