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Chapter 13: Interfaces, Protocols, and ABCs — Notes

1. Three Kinds of Interfaces in Python

Approach Type Enforced At Inherits From Description
Duck Typing Dynamic Runtime object "If it walks like a duck." Works blindly at runtime. Crashes if the method is missing.
Goose Typing Dynamic Runtime abc.ABC Explicitly subclassing an Abstract Base Class. Fails early (at instantiation) if methods are missing.
Static Duck Typing Static Pre-runtime object Typing hints using typing.Protocol. Validates structural compatibility via Mypy before the code ever runs.

2. Abstract Base Classes (ABCs)

  • Found in collections.abc and numbers.
  • You use them to verify interfaces at runtime (isinstance(obj, abc.MutableSequence)).
  • Rule of Thumb: You should subclass existing ABCs frequently. You should almost never create your own custom ABCs unless you are building a massive framework (like Django or Pytest). Custom ABCs introduce rigid coupling.

3. typing.Protocol

  • Introduced in PEP 544.
  • It solves the "Static Duck Typing" problem. If you want Mypy to verify that an object has a .read() method, you don't force the object to inherit from ReaderABC (which forces tight coupling). Instead, you define a Protocol with a .read() method, and Mypy checks if the object structurally matches it.

4. NotImplemented vs NotImplementedError

These sound identical but are completely different:

  • NotImplemented is a singleton (like None). It is returned (not raised) by special methods like __eq__ or __add__ to tell Python: "I don't know how to handle this, try asking the other operand."
  • NotImplementedError is an exception. It is raised in abstract methods to tell the subclass: "You forgot to implement this method!"

5. Virtual Subclasses

You can bypass inheritance entirely using MyABC.register(MyClass). This is called a virtual subclass. isinstance(MyClass(), MyABC) will return True, but Python will NOT check if the methods actually exist, nor will MyClass inherit any mixin methods from the ABC. It is a pure "trust me" mechanism.