<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def first(self, offset):
 """ Convenience method for subsetting initial periods of time series data based on a date offset. Parameters offset : string, DateOffset, dateutil.relativedelta Returns ------- subset : same type as caller Raises ------ TypeError If the index is not a :class:`DatetimeIndex` See Also -------- last : Select final periods of time series based on a date offset. at_time : Select values at a particular time of the day. between_time : Select values between particular times of the day. Examples -------- A 2018-04-09 1 2018-04-11 2 2018-04-13 3 2018-04-15 4 Get the rows for the first 3 days: A 2018-04-09 1 2018-04-11 2 Notice the data for 3 first calender days were returned, not the first 3 days observed in the dataset, and therefore data for 2018-04-13 was not returned. """

        if not isinstance(self.index, DatetimeIndex):
            raise TypeError("'first' only supports a DatetimeIndex index")

        if len(self.index) == 0:
            return self

        offset = to_offset(offset)
        end_date = end = self.index[0] + offset

        # Tick-like, e.g. 3 weeks
        if not offset.isAnchored() and hasattr(offset, '_inc'):
            if end_date in self.index:
                end = self.index.searchsorted(end_date, side='left')
                return self.iloc[:end]

        return self.loc[:end]