List installed packages that APT can't figure out aren't needed

unneeded.py

import apt
import networkx as nx


def is_root(pkg: apt.package.Package) -> bool:
    if not pkg.is_installed:
        return False
    if not pkg.is_auto_installed:
        return True
    if pkg.essential:
        return True
    v = pkg.installed
    if v.priority in ('required', 'important'):
        return True
    return False


def main():
    c = apt.cache.Cache()
    c.open()
    installed = set(name for name in c.keys() if c[name].is_installed)
    deps = nx.DiGraph()
    for name in installed:
        pkg = c[name]
        if is_root(pkg):
            deps.add_edge((), name)
        for dep in pkg.installed.get_dependencies('PreDepends', 'Depends', 'Recommends'):
            for bdep in dep.or_dependencies:
                deps.add_edge(name, bdep.name)
        for vname in pkg.installed.provides:
            deps.add_edge(vname, name)
    needed = set(nx.descendants(deps, ()))
    unneeded = installed - needed
    for pkg in sorted(unneeded):
        print(pkg)


if __name__ == '__main__':
    main()