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Collection protocols

These protocols describe the shared APIs behind built-in collection types.

These protocols describe behavior, not concrete storage. They let the language talk about "something indexable" or "something iterable" without naming one specific collection type.

Core collection protocols

  • Indexed[A(Eq), B]: lookup, assignment, and deletion with []
  • Sliceable[A] (Indexed[int, A]): slicing with start:stop:step
  • Collection[A] (Iterable[A]): construction from an iterable and len
  • Container[A(Eq)] (Collection[A]): membership with in and not in

Specialized collection protocols

  • Sequence[A] (Sliceable[A], Collection[A], Times[int]): ordered, sliceable collections with repetition and mutating operations such as append, insert, and reverse
  • Mapping[A(Eq), B] (Container[A], Indexed[A, B]): key/value collections with get, pop, keys, values, and items
  • Set[A(Eq)] (Container[A], Ord, Logical, Minus): set operations, ordering, membership, and mutating updates such as add and discard

Built-in types make use of these protocols:

  • list[A] implements Sequence[A]
  • dict[A(Hashable), B] implements Mapping[A, B]
  • set[A(Hashable)] implements Set[A]

The protocol surface is sometimes looser than a concrete type's full requirements. For example, Mapping[A, B] talks about key/value behavior in general, while dict specifically requires hashable keys.