What are abstract base classes and Protocols?

Question presented to candidate: "Python lets you describe an interface in two ways: abstract base classes from abc, and Protocol from typing. What is each one, how do they differ, and when would you pick one over the other?"

What a strong answer should cover:

• An ABC (abc.ABC, metaclass ABCMeta) is a base class that declares @abstractmethods; a subclass that does not implement all of them cannot be instantiated, and the check happens at instantiation time, at run time.
• ABCs are nominal: a class belongs by inheriting or by register() (a virtual subclass, unchecked); __subclasshook__ lets an ABC such as collections.abc.Iterable accept any class with the right methods.
• A typing.Protocol (PEP 544, Python 3.8) is structural: any class with matching members satisfies it without inheriting; it is checked by static type checkers such as mypy and pyright, not at run time.
• @runtime_checkable allows isinstance() against a protocol, but it only checks that the member names exist, not their signatures or types; 3.12 changed the lookup to inspect.getattr_static.
• Trade-off: ABCs for frameworks you own that want enforcement and shared concrete methods; Protocols for describing what a function needs from objects you do not control, i.e. typed duck typing.

Code / implementation expected: Yes — an ABC that refuses to instantiate an incomplete subclass, and a Protocol matched by unrelated classes, with the runtime_checkable limits shown.