Lookup an Equity by its ticker symbol.

        Parameters
        ----------
        symbol_str : str
            The ticker symbol for the equity to lookup.
        country_code : str or None, optional
            A country to limit symbol searches to.

        Returns
        -------
        equity : Equity
            The equity that held the ticker symbol on the current
            symbol lookup date.

        Raises
        ------
        SymbolNotFound
            Raised when the symbols was not held on the current lookup date.

        See Also
        --------
        :func:`zipline.api.set_symbol_lookup_date`

def symbol(self, symbol_str, country_code=None):
        """Lookup an Equity by its ticker symbol.

        Parameters
        ----------
        symbol_str : str
            The ticker symbol for the equity to lookup.
        country_code : str or None, optional
            A country to limit symbol searches to.

        Returns
        -------
        equity : Equity
            The equity that held the ticker symbol on the current
            symbol lookup date.

        Raises
        ------
        SymbolNotFound
            Raised when the symbols was not held on the current lookup date.

        See Also
        --------
        :func:`zipline.api.set_symbol_lookup_date`
        """
        # If the user has not set the symbol lookup date,
        # use the end_session as the date for symbol->sid resolution.
        _lookup_date = self._symbol_lookup_date \
            if self._symbol_lookup_date is not None \
            else self.sim_params.end_session

        return self.asset_finder.lookup_symbol(
            symbol_str,
            as_of_date=_lookup_date,
            country_code=country_code,
        )