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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def line_items(self):
 """Apply a datetime filter against the contributors's line item queryset."""

        if self._line_items is None:
            self._line_items = self.contributor.line_items.filter(
                payment_date__range=(self.start, self.end)
            )
        return self._line_items