@abc.abstractmethod
def expression(self, instrument, field, start_time=None, end_time=None, freq='day'):
    'Get Expression data.\n\n        Parameters\n        ----------\n        instrument : str\n            a certain instrument\n        field : str\n            a certain field of feature\n        start_time : str\n            start of the time range\n        end_time : str\n            end of the time range\n        freq : str\n            time frequency, available: year/quarter/month/week/day\n\n        Returns\n        -------\n        pd.Series\n            data of a certain expression\n        '
    raise NotImplementedError('Subclass of ExpressionProvider must implement `Expression` method')
Get Expression data.

Parameters
----------
instrument : str
    a certain instrument
field : str
    a certain field of feature
start_time : str
    start of the time range
end_time : str
    end of the time range
freq : str
    time frequency, available: year/quarter/month/week/day

Returns
-------
pd.Series
    data of a certain expression
@abc.abstractmethod
def expression(self, instrument, field, start_time=None, end_time=None, freq='day'):
    'Get Expression data.\n\n        Parameters\n        ----------\n        instrument : str\n            a certain instrument\n        field : str\n            a certain field of feature\n        start_time : str\n            start of the time range\n        end_time : str\n            end of the time range\n        freq : str\n            time frequency, available: year/quarter/month/week/day\n\n        Returns\n        -------\n        pd.Series\n            data of a certain expression\n        '
    raise NotImplementedError('Subclass of ExpressionProvider must implement `Expression` method')