When would you use a Protocol instead of an abstract base class in Python?

Chooses structural or nominal contracts based on ownership, runtime identity, shared behavior, and third-party integration.

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A Protocol describes the members a type must have for static structural typing. A class can satisfy it without inheriting from or even knowing about the protocol, which is useful for adapters and third-party types. An abstract base class is a nominal runtime relationship: implementations normally inherit or register and can share concrete behavior, class-level invariants, or an explicit family identity. I choose based on whether compatible behavior is enough or membership in the hierarchy matters.

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Both mechanisms can describe behavior, but they differ in how implementations opt in and what the runtime knows.
A Protocol describes the members a type must have for static structural typing. A class can satisfy it without inheriting from or even knowing about the protocol, which is useful for adapters and third-party types. An abstract base class is a nominal runtime relationship: implementations normally inherit or register and can share concrete behavior, class-level invariants, or an explicit family identity. I choose based on whether compatible behavior is enough or membership in the hierarchy matters.
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Protocols keep consumers decoupled from implementation ownership and fit Python's duck-typed style while giving a checker a precise contract. ABCs are useful when the base controls construction, supplies reusable implementation, or runtime relationships are part of the design. A runtime-checkable protocol supports limited runtime checks, but those checks primarily verify attribute presence rather than full signatures or semantic behavior, so it is not a validation system.

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Saying protocols enforce behavior at runtime by default.
Using an ABC only because an interface-like keyword feels familiar.
Creating a large protocol that mirrors an entire vendor client.
Assuming matching method names guarantee matching semantics.

When would you use a Protocol instead of an abstract base class in Python?

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