Ask HN: How to build minimal login and payment for web app?

What is the easiest way to build a minimal product that has a way to have users sign in and pay a monthly subscription? Let’s say we want to do this in Python. To make this more concrete suppose I want to monetize a function hello(name) that takes a string and prints out "Hello, " + name + "!". What is the easiest way to build the subscription and payment infrastructure? Is there a SaaS that solves this problem?