chapter_13_interfaces_protocols_abcs

Chapter 13 — Interfaces, Protocols, and ABCs

Theme: Python evolved from pure runtime Duck Typing to explicit Goose Typing (ABCs), and finally to robust Static Duck Typing (Protocols). Understanding when to use which is the hallmark of a senior Python engineer.

What You'll Learn

Paradigm	Concept	When to use
Duck Typing	Implicit runtime execution (hasattr)	General dynamic scripting, quick tooling.
Goose Typing	Explicit runtime validation (abc.ABC)	Building frameworks where failing fast is critical.
Static Duck Typing	Implicit static validation (typing.Protocol)	Modern codebases with Mypy, dependency injection.
Monkey Patching	Altering class structures at runtime	Fixing third-party libraries, test mocking.
Virtual Subclasses	ABC.register(Class)	Mapping legacy code into modern ABC hierarchies.

Key Files

• examples.py — Side-by-side implementations of the three typing paradigms and a custom Tombola ABC.
• exercises.py — Monkey patching __setitem__ to fix random.shuffle() and registering virtual subclasses.
• mini_project.py — A notification plugin system demonstrating why Protocol is superior to inheritance for loose coupling.
• benchmarks.py — Proves that isinstance(obj, ABC) is ~4x slower than a simple hasattr() check.
• notes.md — The differences between NotImplemented and NotImplementedError.
• pitfalls.md — The dangers of over-engineering custom ABCs and catching NotImplementedError.
• interview_questions.md — L3 to L6 interview prep.
• architecture_notes.md — CPython's __subclasshook__ and the ABCMeta metaclass.

30-Second Rules

# Rule 1: Do not create custom ABCs unless you are writing a framework.
# Instead, subclass the built-in ABCs in `collections.abc`.
from collections.abc import MutableSequence
class MyList(MutableSequence): ...

# Rule 2: If you want Mypy to verify an interface WITHOUT forcing inheritance, use Protocol.
from typing import Protocol
class Renderable(Protocol):
    def render(self) -> str: ...

# Rule 3: `NotImplemented` is a singleton you RETURN. 
# `NotImplementedError` is an exception you RAISE.
class ABC:
    def method(self):
        raise NotImplementedError  # Tell the subclass they missed it!

Reference: Fluent Python 2nd ed., Chapter 13 — pages 459–504