Protocol - structural vs nominal subtyping

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Protocol implements structural subtyping, sometimes called 'static duck typing'. The type checker asks 'does this object have the required attributes and methods?' - not 'does it inherit from the right class?'. This is the critical difference from ABC (Abstract Base Class), which uses nominal subtyping: a class must explicitly register or inherit. With Protocol, the third-party class satisfies Drawable just by having a compatible draw() method, even if its author never heard of your Drawable Protocol. This makes Protocol ideal for libraries: you define the interface you need, and any conforming object works without requiring callers to subclass your types. For runtime checks, a Protocol decorated with @runtime_checkable also supports isinstance(). Without that decorator, Protocol is purely static. The mental model: ABC is 'is-a' checked by inheritance; Protocol is 'looks-like' checked by shape. In Python's duck-typing culture, Protocol fits more naturally.