Protocol vs ABC - the registration difference

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Without @runtime_checkable, a Protocol is a purely static construct. Passing it to isinstance() at runtime raises TypeError. This surprises many developers who expect structural checking to work at runtime the same way it works in mypy. It does not. Adding @runtime_checkable to the Protocol class enables isinstance() checks, but the check is still shallow - it only verifies that the required method names exist on the object, not that their signatures are compatible. The practical takeaway: use Protocol for static analysis everywhere, but if you genuinely need runtime structural checks, add @runtime_checkable and be aware its guarantees are weaker than mypy's static guarantees.