Given a docname path, pick apart and return name of parent

def parse_parent(docname):
    """ Given a docname path, pick apart and return name of parent """

    lineage = docname.split('/')
    lineage_count = len(lineage)

    if docname == 'index':
        # This is the top of the Sphinx project
        parent = None
    elif lineage_count == 1:
        # This is a non-index doc in root, e.g. about
        parent = 'index'
    elif lineage_count == 2 and lineage[-1] == 'index':
        # This is blog/index, parent is the root
        parent = 'index'
    elif lineage_count == 2:
        # This is blog/about
        parent = lineage[0] + '/index'
    elif lineage[-1] == 'index':
        # This is blog/sub/index
        parent = '/'.join(lineage[:-2]) + '/index'
    else:
        # This should be blog/sub/about
        parent = '/'.join(lineage[:-1]) + '/index'

    return parent