'Lookup an Equity by its ticker symbol.
Parameters
symbol_str : str
The ticker symbol for the equity to lookup.
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`'

@api_method
@preprocess(symbol_str=ensure_upper_case)
def symbol(self, symbol_str):

    _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)
