def _get_source_index(source_usage_key, source_parent):
    """
    Get source index position of the XBlock.

    Arguments:
        source_usage_key (BlockUsageLocator): Locator of source item.
        source_parent (XBlock): A parent of the source XBlock.

    Returns:
        source_index (int): Index position of the xblock in a parent.
    """
    try:
        source_index = source_parent.children.index(source_usage_key)
        return source_index
    except ValueError:
        return None