If someone was intent on making a working cross platform Python virus, i would expect it should be just a matter of adding a few conditional switches to execute the different infection approaches depending on which system it is running on (and to give it a polish, check the effectiveness and try different approaches when the most commonly successful one for the system in question fails; and of course before the actual attack on the OS spread a bunch of modified and obfuscated copies as backup in case the original one gets caught)