Creates an index on this collection.

        Takes either a single key or a list of (key, direction) pairs.
        The key(s) must be an instance of :class:`basestring`
        (:class:`str` in python 3), and the direction(s) must be one of
        (:data:`~pymongo.ASCENDING`, :data:`~pymongo.DESCENDING`,
        :data:`~pymongo.GEO2D`, :data:`~pymongo.GEOHAYSTACK`,
        :data:`~pymongo.GEOSPHERE`, :data:`~pymongo.HASHED`,
        :data:`~pymongo.TEXT`).

        To create a single key ascending index on the key ``'mike'`` we just
        use a string argument::

          >>> my_collection.create_index("mike")

        For a compound index on ``'mike'`` descending and ``'eliot'``
        ascending we need to use a list of tuples::

          >>> my_collection.create_index([("mike", pymongo.DESCENDING),
          ...                             ("eliot", pymongo.ASCENDING)])

        All optional index creation parameters should be passed as
        keyword arguments to this method. For example::

          >>> my_collection.create_index([("mike", pymongo.DESCENDING)],
          ...                            background=True)

        Valid options include, but are not limited to:

          - `name`: custom name to use for this index - if none is
            given, a name will be generated.
          - `unique`: if ``True`` creates a uniqueness constraint on the index.
          - `background`: if ``True`` this index should be created in the
            background.
          - `sparse`: if ``True``, omit from the index any documents that lack
            the indexed field.
          - `bucketSize`: for use with geoHaystack indexes.
            Number of documents to group together within a certain proximity
            to a given longitude and latitude.
          - `min`: minimum value for keys in a :data:`~pymongo.GEO2D`
            index.
          - `max`: maximum value for keys in a :data:`~pymongo.GEO2D`
            index.
          - `expireAfterSeconds`: <int> Used to create an expiring (TTL)
            collection. MongoDB will automatically delete documents from
            this collection after <int> seconds. The indexed field must
            be a UTC datetime or the data will not expire.
          - `partialFilterExpression`: A document that specifies a filter for
            a partial index.
          - `collation` (optional): An instance of
            :class:`~pymongo.collation.Collation`. This option is only supported
            on MongoDB 3.4 and above.

        See the MongoDB documentation for a full list of supported options by
        server version.

        .. warning:: `dropDups` is not supported by MongoDB 3.0 or newer. The
          option is silently ignored by the server and unique index builds
          using the option will fail if a duplicate value is detected.

        .. note:: `partialFilterExpression` requires server version **>= 3.2**

        .. note:: The :attr:`~pymongo.collection.Collection.write_concern` of
           this collection is automatically applied to this operation when using
           MongoDB >= 3.4.

        :Parameters:
          - `keys`: a single key or a list of (key, direction)
            pairs specifying the index to create
          - `**kwargs` (optional): any additional index creation
            options (see the above list) should be passed as keyword
            arguments

        .. versionchanged:: 3.4
           Apply this collection's write concern automatically to this operation
           when connected to MongoDB >= 3.4. Support the `collation` option.
        .. versionchanged:: 3.2
            Added partialFilterExpression to support partial indexes.
        .. versionchanged:: 3.0
            Renamed `key_or_list` to `keys`. Removed the `cache_for` option.
            :meth:`create_index` no longer caches index names. Removed support
            for the drop_dups and bucket_size aliases.

        .. mongodoc:: indexes

def create_index(self, keys, **kwargs):
        """Creates an index on this collection.

        Takes either a single key or a list of (key, direction) pairs.
        The key(s) must be an instance of :class:`basestring`
        (:class:`str` in python 3), and the direction(s) must be one of
        (:data:`~pymongo.ASCENDING`, :data:`~pymongo.DESCENDING`,
        :data:`~pymongo.GEO2D`, :data:`~pymongo.GEOHAYSTACK`,
        :data:`~pymongo.GEOSPHERE`, :data:`~pymongo.HASHED`,
        :data:`~pymongo.TEXT`).

        To create a single key ascending index on the key ``'mike'`` we just
        use a string argument::

          >>> my_collection.create_index("mike")

        For a compound index on ``'mike'`` descending and ``'eliot'``
        ascending we need to use a list of tuples::

          >>> my_collection.create_index([("mike", pymongo.DESCENDING),
          ...                             ("eliot", pymongo.ASCENDING)])

        All optional index creation parameters should be passed as
        keyword arguments to this method. For example::

          >>> my_collection.create_index([("mike", pymongo.DESCENDING)],
          ...                            background=True)

        Valid options include, but are not limited to:

          - `name`: custom name to use for this index - if none is
            given, a name will be generated.
          - `unique`: if ``True`` creates a uniqueness constraint on the index.
          - `background`: if ``True`` this index should be created in the
            background.
          - `sparse`: if ``True``, omit from the index any documents that lack
            the indexed field.
          - `bucketSize`: for use with geoHaystack indexes.
            Number of documents to group together within a certain proximity
            to a given longitude and latitude.
          - `min`: minimum value for keys in a :data:`~pymongo.GEO2D`
            index.
          - `max`: maximum value for keys in a :data:`~pymongo.GEO2D`
            index.
          - `expireAfterSeconds`: <int> Used to create an expiring (TTL)
            collection. MongoDB will automatically delete documents from
            this collection after <int> seconds. The indexed field must
            be a UTC datetime or the data will not expire.
          - `partialFilterExpression`: A document that specifies a filter for
            a partial index.
          - `collation` (optional): An instance of
            :class:`~pymongo.collation.Collation`. This option is only supported
            on MongoDB 3.4 and above.

        See the MongoDB documentation for a full list of supported options by
        server version.

        .. warning:: `dropDups` is not supported by MongoDB 3.0 or newer. The
          option is silently ignored by the server and unique index builds
          using the option will fail if a duplicate value is detected.

        .. note:: `partialFilterExpression` requires server version **>= 3.2**

        .. note:: The :attr:`~pymongo.collection.Collection.write_concern` of
           this collection is automatically applied to this operation when using
           MongoDB >= 3.4.

        :Parameters:
          - `keys`: a single key or a list of (key, direction)
            pairs specifying the index to create
          - `**kwargs` (optional): any additional index creation
            options (see the above list) should be passed as keyword
            arguments

        .. versionchanged:: 3.4
           Apply this collection's write concern automatically to this operation
           when connected to MongoDB >= 3.4. Support the `collation` option.
        .. versionchanged:: 3.2
            Added partialFilterExpression to support partial indexes.
        .. versionchanged:: 3.0
            Renamed `key_or_list` to `keys`. Removed the `cache_for` option.
            :meth:`create_index` no longer caches index names. Removed support
            for the drop_dups and bucket_size aliases.

        .. mongodoc:: indexes
        """
        keys = helpers._index_list(keys)
        name = kwargs.setdefault("name", helpers._gen_index_name(keys))
        self.__create_index(keys, kwargs)
        return name