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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 _get_db():
 """ Returns the connection to the database using the settings. This function should not be called outside of this file. Use db instead. """

    from .settings import settings
    mongo = settings.MONGODB

    if 'URI' in mongo and mongo['URI']:
        uri = mongo['URI']
    else:
        uri = 'mongodb://'

        if all(mongo.get(key) for key in ('USERNAME', 'PASSWORD')):
            uri += '{0}:{1}@'.format(mongo['USERNAME'], mongo['PASSWORD'])

        if 'HOSTS' in mongo and mongo['HOSTS']:
            uri += ','.join(
                '{0}:{1}'.format(host, port)
                for (host, port) in zip(mongo['HOSTS'], mongo['PORTS']),
            )
        else:
            uri += '{0}:{1}'.format(mongo['HOST'], mongo.get('PORT', 27017))

        uri += '/' + mongo['DATABASE']

        if 'OPTIONS' in mongo and mongo['OPTIONS']:
            uri += '?{0}'.format('&'.join(mongo['OPTIONS']))

    client = ConnectionFailureProxy(MongoClient(uri, connect=False))
    database = client[parse_uri(uri)['database']]

    return database