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).

def default_returns_func(symbol, start=None, end=None):
    """
    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).
    """

    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]