Hello all,
I'm really a beginner in python programming and I'd like to launch a python function from an action. I need, from the partner tree, to select partners in the list then, with the action, call a function to modify each of them. I know howto open a form but doesn't know howto call a function directly.

Thanks for your help

you can create the function def in the specified class (def) which does changes you want, this in the python file, and for the action in the view call the function with the on_change attribute :
example :
sale_view line 107 :
field name="partner_id" on_change="onchange_partner_id(partner_id)"
call function in sale.py line 311 :
def onchange_partner_id(self, cr, uid, ids, part): .....

a little description in french for object and view :
http://myopenerp.blogspot.com/2009/12/les-objets-en-details.html