Let me be more clear. In the res.partner.form view the user can tick or untick the checkbox which says that the contact is a 'Vendor' (by default the model calls this field 'supplier' and it is a boolean). I want to be able to set the value of a custom char field to 'whatever' if the 'supplier' boolean is set to true, otherwise I want that char field empty. This is all just a stepping stone for me to eventually be able to do something more complex but I'll omit that as I don't want to add complexity here.

So far I know that I can use automated actions to call a server action when the 'supplier' field is changed. I don't know how to write the python code for the server action. Could I please get some pointers on this?