def maybe_mongo(uri):
    """
    Check whether arg should be regarded as MongoDB

    Prameters
    ---------
    uri: obj
       Argument to be distinguished

    Returns
    -------
    bool: maybe_mongo
    """
    try:
        import pymongo
    except ImportError:
        return False
    if isinstance(uri, (pymongo.mongo_client.MongoClient, pymongo.
        collection.Collection)):
        msg = (
            'To initialize MongoBackend, pymongo Database instance must be provided, given: {}{}'
            )
        raise ValueError(msg.format(uri, type(uri)))
    elif isinstance(uri, pymongo.database.Database):
        return True
    elif not isinstance(uri, str):
        return False
    return uri.startswith('mongodb://')
