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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 ensure(self, collection):
        ''' Call the pymongo method ``ensure_index`` on the passed collection.

            :param collection: the ``pymongo`` collection to ensure this index \
                    is on
        '''
        components = []
        for c in self.components:
            if isinstance(c[0], Field):
                c = (c[0].db_field, c[1])
            components.append(c)

        extras = {}
        if self.__min is not None:
            extras['min'] = self.__min
        if self.__max is not None:
            extras['max'] = self.__max
        if self.__bucket_size is not None:
            extras['bucket_size'] = self.__bucket_size
        if self.__expire_after is not None:
            extras['expireAfterSeconds'] = self.__expire_after

        collection.ensure_index(components, unique=self.__unique,
            drop_dups=self.__drop_dups, **extras)
        return self