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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 count(self, *, page_size=DEFAULT_BATCH_SIZE, **options):
 """Counts the number of entities that match this query. Note: Since Datastore doesn't provide a native way to count entities by query, this method paginates through all the entities' keys and counts them. Parameters: \**options(QueryOptions, optional) Returns: int: The number of entities. """

        entities = 0
        options = QueryOptions(self).replace(keys_only=True)
        for page in self.paginate(page_size=page_size, **options):
            entities += len(list(page))
        return entities