How do I update my account list in python as soon as new data is added in mongodb( face recognition project)

Question

I am creating a project which will recognize the faces in real time as soon as you have signed up to my website and your data will be updated in mongo db.

my python server will keep on identifying faces whose list is available to it via mongodb. I need to add new data to my face list whenever new user sign up.

I know I can set up in a loop to check new data every time it execute. But if some one know how to run a python program to update the list whenever a user finishes a sign up; so that the python program will update my face list in other python progrm to detect faces.