I don't think the "/usr/bin/env" case needs to limit qualified names like "python3[.x]" to registered installations. This is a choice made to simplify the implementation. If it finds a generic "python.exe" executable in PATH, for 3.5+ it is possible to query version information directly from it. Older versions lack this metadata, but it's also possible to inspect the PE import table (i.e. IMAGE_DIRECTORY_ENTRY_IMPORT) for a dependency on "python2x.dll" or "python3x.dll".