@abc.abstractmethod
def dataset(self, instruments, fields, start_time=None, end_time=None, freq='day'):
    'Get dataset data.\n\n        Parameters\n        ----------\n        instruments : list or dict\n            list/dict of instruments or dict of stockpool config\n        fields : list\n            list of feature instances\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\n\n        Returns\n        ----------\n        pd.DataFrame\n            a pandas dataframe with <instrument, datetime> index\n        '
    raise NotImplementedError('Subclass of DatasetProvider must implement `Dataset` method')
Get dataset data.

Parameters
----------
instruments : list or dict
    list/dict of instruments or dict of stockpool config
fields : list
    list of feature instances
start_time : str
    start of the time range
end_time : str
    end of the time range
freq : str
    time frequency

Returns
----------
pd.DataFrame
    a pandas dataframe with <instrument, datetime> index
@abc.abstractmethod
def dataset(self, instruments, fields, start_time=None, end_time=None, freq='day'):
    'Get dataset data.\n\n        Parameters\n        ----------\n        instruments : list or dict\n            list/dict of instruments or dict of stockpool config\n        fields : list\n            list of feature instances\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\n\n        Returns\n        ----------\n        pd.DataFrame\n            a pandas dataframe with <instrument, datetime> index\n        '
    raise NotImplementedError('Subclass of DatasetProvider must implement `Dataset` method')