You have Two Option 

First One 
you can use Change method for send notification .....

@api.onchange('unit_price')

def _onchange_price(self):

write your logic 

Second Option is 
you can also use write method for this 

write your logic on write method .... 

Usman, thank you for the answer. Can you tell me where in the web interface I can define these functions?

The approach is clear, but it is not clear where to plug in this function. I am using the standard Odoo app for product management.