100 % portable ?

[Vent d'Est - East Wind]

i have some question about portability of python

for my part i work under the 3 systemes (windows , linux , mac os x 
sometimes mac os classic too )

i want to know how i can be sure at 100 % that my programme is portable
(i dont want code each times i use other system )

i suppose for GUI is same

wich thing need i pay attention (language , system , library .. )
do you have some graphic library to propose (i see GTK , Tkinter and 
WXwindow )

thankx in advance for you reply

East wind