def _s3_path_split(s3_path):
    """
    Split an S3 path into bucket and key.

    Parameters
    ----------
    s3_path : str

    Returns
    -------
    splitted : (str, str)
        (bucket, key)

    Examples
    --------
    >>> _s3_path_split('s3://my-bucket/foo/bar.jpg')
    S3Path(bucket_name='my-bucket', key='foo/bar.jpg')
    """
    if not s3_path.startswith('s3://'):
        raise ValueError('s3_path is expected to start with \'s3://\', '
                         'but was {}'.format(s3_path))
    bucket_key = s3_path[len('s3://'):]
    bucket_name, key = bucket_key.split('/', 1)
    return S3Path(bucket_name, key)