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The reason is that the term iterator protocol,ДЇor example, is widely understood in the community, and coming up withДЄ new term for this concept in a statically typed context would just create We propose to use the term protocols for types supporting structural Type SomeInterface interface Д«oth these ideas are questionable in the context of this proposal.

To mark interface attributes, and to explicitly declare implementation. Special classes to distinguish interface classes from normal classes,

zope.interface was one of the first widely usedДЄpproaches to structural subtyping in Python.

Existing Approaches to Structural SubtypingД«efore describing the actual specification, we review and comment on existingДЄpproaches related to structural subtyping in Python and other languages: To provide a seamless transition for projects that already use ABCs. In the runtime context as ABCs is rather a minor bonus that exists mostly Is not a goal of this PEP, the main goal is to provide a support and standardsДЇor static structural subtyping. To reiterate, providing complex runtime semantics for protocol classes

There is no intent to make protocols non-optional in the future.

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Programmers are free to not use them even if they use type annotations.Any checks will be performed only by third-party type checkers and.No runtime semantics will be imposed for variables or parameters annotated.As well, following PEP 484 and PEP 526 we state that protocols are This would be difficult and error-prone and will contradict the logic Provide sophisticated runtime instance and class checks against protocolД¬lasses. Non-goalsДЄt runtime, protocol classes will be simple ABCs. See section on rejected ideas at theД®nd of this PEP for additional motivation. This PEP complement normal classes, and users are free to choose Instead, protocol classes as specified in Therefore, in this PEP weДo not propose to replace the nominal subtyping described by PEP 484 with Subtyping have their strengths and weaknesses. However, as discussed in PEP 483, both nominal and structural Is treated independently of its actual runtime class. The runtime semantics of duck typing: an object that has certain properties Structural subtyping is natural for Python programmers since it matches With additional, unrelated classes that happen to implement It is also automatically extensible and works

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The above code with a protocol class matches common PythonД¬onventions much better. The same functionality will be provided for user-defined protocols, as The main goal of this proposal is to support such behavior statically. Note that ABCs in typing module already provide structural behaviorДЄt runtime, isinstance(Bucket(), Iterable) returns True. result : int = collect ( Bucket ()) # Passes type check For example,ДЇrom typing import Iterator, Iterable class Bucket. Normally do in idiomatic dynamically typed Python code. To support them, which is unpythonic and unlike what one would The problem with them is that a class has to be explicitly marked Rationale and GoalsД¬urrently, PEP 484 and the typing module define abstractД«ase classes for several common Python protocols such as Iterable and Static and runtime semantics of protocol classes that will provide a supportДЇor structural subtyping (static duck typing). Only specifies the semantics of nominal subtyping. Type hints introduced in PEP 484 can be used to specify type metadataДЇor static type checkers and other third party tools.

Make protocols special objects at runtime rather than normal ABCs.

Call structural base types вЂњinterfacesвЂќ.

Overriding inferred variance of protocol classes.

Covariant subtyping of mutable attributes.

Prohibit explicit subclassing of protocols by non-protocols.

Provide a special intersection type construct.

Use assignments to check explicitly that a class implements a protocol.

Make protocols interoperable with other approaches.

Allow only protocol methods and force use of getters and setters.

Runtime Implementation of Protocol Classes.

decorator and narrowing types by isinstance().

Modules as implementations of protocols.

Subtyping relationships with other types.

Existing Approaches to Structural Subtyping.

Toggle light / dark / auto colour theme PEP 544 вЂ“ Protocols: Structural subtyping (static duck typing) Author : Ivan Levkivskyi, Jukka Lehtosalo, ЕЃukasz Langa BDFL-Delegate : Guido van Rossum Discussions-To : Python-Dev list Status : Accepted Type : Standards Track Created : 0 Python-Version : 3.8 Resolution : Typing-SIG message PEP 544 вЂ“ Protocols: Structural subtyping (static duck typing) | Following system colour scheme Selected dark colour scheme Selected light colour scheme Python Enhancement Proposals