def split_uri(uri, pattern='s3://', separator='/'):
    """Convert a URI to a bucket, object name tuple.

    's3://bucket/path/to/thing' -> ('bucket', 'path/to/thing')
    """
    assert pattern in uri
    parts = uri[len(pattern):].split(separator)
    bucket = parts[0]
    path = separator.join(parts[1:])
    return bucket, path