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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(self):
 """Get the unicode string representing the whole collection."""

        import datetime
        items = []
        time_begin = datetime.datetime.now()
        for href in self.list():
            items.append(self.get(href).item)
        time_end = datetime.datetime.now()
        self.logger.info(
            "Collection read %d items in %s sec from %s", len(items),
            (time_end - time_begin).total_seconds(), self.path)
        if self.get_meta("tag") == "VCALENDAR":
            collection = vobject.iCalendar()
            for item in items:
                for content in ("vevent", "vtodo", "vjournal"):
                    if content in item.contents:
                        for item_part in getattr(item, "%s_list" % content):
                            collection.add(item_part)
                        break
            return collection.serialize()
        elif self.get_meta("tag") == "VADDRESSBOOK":
            return "".join([item.serialize() for item in items])
        return ""