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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 open_spider(self, spider):
 """ Initialize Mongodb client. """

        if self.url == "":
            self.client = pymongo.MongoClient(self.host, self.port)
        else:
            self.client = pymongo.MongoClient(self.url)

        self.db_name, self.collection_name = self._replace_placeholder(spider)
        self.db = self.client[self.db_name]