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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 exists(self, callback=None):
 """ Returns True if the key exists :rtype: bool :return: Whether the key exists on S3 """

        def existence_tested(response):
            if callable(callback):
                callback(bool(response))
        self.bucket.lookup(self.name, callback=existence_tested)