How to call a Python function from an act_window in OpenERP7?

I added a new option to the menu More, at the top side of the screen (tree view). To do that, I did an act_window in XML:

<act_window name="Invite partners to an event"
     res_model="mail.compose.message"
     src_model="event.registration"
     view_mode="form"
     multi="True"
     target="new"
     id="invite_partner_to_event"/>

Everything works OK, I can select rows of the tree view of event.registration, click on More, and mi option Invite partners to an event appears. When I click on this, the form view of the model mail.compose.message is opened in a pop-up, which is exactly what I want.

Now my problem: I'd like to perform a Python function when I click on my option, (and the user mustn't notice that). I.E.: the user won't be able to feel the difference, but I will be modifying several things in the backend through that function.

¿How could I manage this? Thank you in advance.