Most Python developers know that __len__ makes len() work and __add__ makes + work. That is the surface. The actual story is that Python's data model is a coherent, documented protocol through which user-defined objects can participate in the language itself: not just operators, but truthiness, hashing, iteration, context management, attribute access, memory layout, and more. Using it well means understanding what CPython actually calls, in what order, and why. The Interpreter Calls These, Not You The first thing to internalize: dunder methods are called by the interpreter, not by user code. When you write len(obj) , Python calls type(obj).__len__(obj) . Not obj.__len__() . The lookup goes through the type, not the instance. This matters for two reasons. First, defining a dunder on an instance rather than the class does not work: class MyClass : pass obj = MyClass () obj .…