'Create a new instance of :class:`GridFS`.
Raises :class:`TypeError` if `database` is not an instance of
:class:`~pymongo.database.Database`.
:Parameters:
- `database`: database to use
- `collection` (optional): root collection to use
.. versionchanged:: 3.1
Indexes are only ensured on the first write to the DB.
.. versionchanged:: 3.0
`database` must use an acknowledged
:attr:`~pymongo.database.Database.write_concern`
.. mongodoc:: gridfs'

def __init__(self, database, collection='fs'):

    if (not isinstance(database, Database)):
        raise TypeError('database    must    be    an    instance    of    Database')
    if (not database.write_concern.acknowledged):
        raise ConfigurationError('database    must    use    acknowledged    write_concern')
    self.__database = database
    self.__collection = database[collection]
    self.__files = self.__collection.files
    self.__chunks = self.__collection.chunks
