def __init__(self, returns):
if not isinstance(returns, pd.DataFrame):<EOL><INDENT>raise TypeError("<STR_LIT>")<EOL><DEDENT>self.returns = returns<EOL>tickers = list(returns.columns)<EOL>super().__init__(len(tickers), tickers)<EOL>
:param returns: asset historical returns
:type returns: pd.DataFrame
:raises TypeError: if ``returns`` is not a dataframe