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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 clean_publish_dates(self):
 """ If an end_date value is provided, the start_date must be less. """

        if self.end_date:
            if not self.start_date:
                raise ValidationError("""The End Date requires a Start Date value.""")
            elif self.end_date <= self.start_date:
                raise ValidationError("""The End Date must not precede the Start Date.""")