[Zope] import of "aq_base" from "Acquisition" is unauthorized. You are not allowed to access 'aq_base' in this context

[KLEIN Stéphane]

I've add "from Acquisition import aq_base" in python script in 
"portal_skins/custom/"

When I test a page, I've this error :

import of "aq_base" from "Acquisition" is unauthorized. You are not 
allowed to access 'aq_base' in this context

Why this ? How to resolve it ?

I search information about it in Zope Api and Google, I found nothing :(

Thanks for your help,
Stéphane