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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 serialize_date(attr, **kwargs):
 """Serialize Date object into ISO-8601 formatted string. :param Date attr: Object to be serialized. :rtype: str """

        if isinstance(attr, str):
            attr = isodate.parse_date(attr)
        t = "{:04}-{:02}-{:02}".format(attr.year, attr.month, attr.day)
        return t