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itself","anchor":"calling-the-hook-on-the-class-itself","htmlText":"Calling the hook on the class itself"},{"level":3,"text":"Other variants of calling the hooks","anchor":"other-variants-of-calling-the-hooks","htmlText":"Other variants of calling the hooks"},{"level":3,"text":"Requiring an explicit decorator on __init_subclass__","anchor":"requiring-an-explicit-decorator-on-__init_subclass__","htmlText":"Requiring an explicit decorator on __init_subclass__"},{"level":3,"text":"A more __new__-like hook","anchor":"a-more-__new__-like-hook","htmlText":"A more __new__-like hook"},{"level":3,"text":"Adding a class attribute with the attribute order","anchor":"adding-a-class-attribute-with-the-attribute-order","htmlText":"Adding a class attribute with the attribute order"},{"level":2,"text":"History","anchor":"history","htmlText":"History"},{"level":2,"text":"References","anchor":"references","htmlText":"References"},{"level":2,"text":"Copyright","anchor":"copyright","htmlText":"Copyright"}],"lineInfo":{"truncatedLoc":"473","truncatedSloc":"354"},"mode":"file"},"image":false,"isCodeownersFile":null,"isPlain":false,"isValidLegacyIssueTemplate":false,"issueTemplate":null,"discussionTemplate":null,"language":"reStructuredText","languageID":419,"large":false,"planSupportInfo":{"repoIsFork":null,"repoOwnedByCurrentUser":null,"requestFullPath":"/python/peps/blob/main/peps/pep-0487.rst","showFreeOrgGatedFeatureMessage":null,"showPlanSupportBanner":null,"upgradeDataAttributes":null,"upgradePath":null},"publishBannersInfo":{"dismissActionNoticePath":"/settings/dismiss-notice/publish_action_from_dockerfile","releasePath":"/python/peps/releases/new?marketplace=true","showPublishActionBanner":false},"rawBlobUrl":"https://github.com/python/peps/raw/refs/heads/main/peps/pep-0487.rst","renderImageOrRaw":false,"richText":"\u003carticle class=\"markdown-body entry-content container-lg\" itemprop=\"text\"\u003e\u003cp dir=\"auto\"\u003ePEP: 487\nTitle: Simpler customisation of class creation\nAuthor: Martin Teichmann \u0026lt;\u003ca href=\"mailto:lkb.teichmann@gmail.com\"\u003elkb.teichmann@gmail.com\u003c/a\u003e\u0026gt;,\nStatus: Final\nType: Standards Track\nCreated: 27-Feb-2015\nPython-Version: 3.6\nPost-History: 27-Feb-2015, 05-Feb-2016, 24-Jun-2016, 02-Jul-2016, 13-Jul-2016\nReplaces: 422\nResolution: \u003ca href=\"https://mail.python.org/pipermail/python-dev/2016-July/145629.html\" rel=\"nofollow\"\u003ehttps://mail.python.org/pipermail/python-dev/2016-July/145629.html\u003c/a\u003e\u003c/p\u003e\n\u003ca name=\"user-content-abstract\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eAbstract\u003c/h2\u003e\u003ca id=\"user-content-abstract\" class=\"anchor\" aria-label=\"Permalink: Abstract\" href=\"#abstract\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eCurrently, customising class creation requires the use of a custom metaclass.\nThis custom metaclass then persists for the entire lifecycle of the class,\ncreating the potential for spurious metaclass conflicts.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThis PEP proposes to instead support a wide range of customisation\nscenarios through a new \u003ccode\u003e__init_subclass__\u003c/code\u003e hook in the class body,\nand a hook to initialize attributes.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe new mechanism should be easier to understand and use than\nimplementing a custom metaclass, and thus should provide a gentler\nintroduction to the full power of Python's metaclass machinery.\u003c/p\u003e\n\u003ca name=\"user-content-background\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eBackground\u003c/h2\u003e\u003ca id=\"user-content-background\" class=\"anchor\" aria-label=\"Permalink: Background\" href=\"#background\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eMetaclasses are a powerful tool to customize class creation. They have,\nhowever, the problem that there is no automatic way to combine metaclasses.\nIf one wants to use two metaclasses for a class, a new metaclass combining\nthose two needs to be created, typically manually.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThis need often occurs as a surprise to a user: inheriting from two base\nclasses coming from two different libraries suddenly raises the necessity\nto manually create a combined metaclass, where typically one is not\ninterested in those details about the libraries at all. This becomes\neven worse if one library starts to make use of a metaclass which it\nhas not done before. While the library itself continues to work perfectly,\nsuddenly every code combining those classes with classes from another library\nfails.\u003c/p\u003e\n\u003ca name=\"user-content-proposal\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eProposal\u003c/h2\u003e\u003ca id=\"user-content-proposal\" class=\"anchor\" aria-label=\"Permalink: Proposal\" href=\"#proposal\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWhile there are many possible ways to use a metaclass, the vast majority\nof use cases falls into just three categories: some initialization code\nrunning after class creation, the initialization of descriptors and\nkeeping the order in which class attributes were defined.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe first two categories can easily be achieved by having simple hooks\ninto the class creation:\u003c/p\u003e\n\u003col dir=\"auto\"\u003e\n\u003cli\u003eAn \u003ccode\u003e__init_subclass__\u003c/code\u003e hook that initializes\nall subclasses of a given class.\u003c/li\u003e\n\u003cli\u003eupon class creation, a \u003ccode\u003e__set_name__\u003c/code\u003e hook is called on all the\nattribute (descriptors) defined in the class, and\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp dir=\"auto\"\u003eThe third category is the topic of another PEP, \u003ca href=\"http://www.python.org/dev/peps/pep-0520\" rel=\"nofollow\"\u003ePEP 520\u003c/a\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAs an example, the first use case looks as follows:\u003c/p\u003e\n\u003cpre\u003e\u0026gt;\u0026gt;\u0026gt; class QuestBase:\n... # this is implicitly a @classmethod (see below for motivation)\n... def __init_subclass__(cls, swallow, **kwargs):\n... cls.swallow = swallow\n... super().__init_subclass__(**kwargs)\n\n\u0026gt;\u0026gt;\u0026gt; class Quest(QuestBase, swallow=\"african\"):\n... pass\n\n\u0026gt;\u0026gt;\u0026gt; Quest.swallow\n'african'\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eThe base class \u003ccode\u003eobject\u003c/code\u003e contains an empty \u003ccode\u003e__init_subclass__\u003c/code\u003e\nmethod which serves as an endpoint for cooperative multiple inheritance.\nNote that this method has no keyword arguments, meaning that all\nmethods which are more specialized have to process all keyword\narguments.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThis general proposal is not a new idea (it was first suggested for\ninclusion in the language definition \u003ca href=\"https://mail.python.org/pipermail/python-dev/2001-November/018651.html\" rel=\"nofollow\"\u003emore than 10 years ago\u003c/a\u003e, and a\nsimilar mechanism has long been supported by \u003ca href=\"http://docs.zope.org/zope_secrets/extensionclass.html\" rel=\"nofollow\"\u003eZope's ExtensionClass\u003c/a\u003e),\nbut the situation has changed sufficiently in recent years that\nthe idea is worth reconsidering for inclusion.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe second part of the proposal adds an \u003ccode\u003e__set_name__\u003c/code\u003e\ninitializer for class attributes, especially if they are descriptors.\nDescriptors are defined in the body of a\nclass, but they do not know anything about that class, they do not\neven know the name they are accessed with. They do get to know their\nowner once \u003ccode\u003e__get__\u003c/code\u003e is called, but still they do not know their\nname. This is unfortunate, for example they cannot put their\nassociated value into their object's \u003ccode\u003e__dict__\u003c/code\u003e under their name,\nsince they do not know that name. This problem has been solved many\ntimes, and is one of the most important reasons to have a metaclass in\na library. While it would be easy to implement such a mechanism using\nthe first part of the proposal, it makes sense to have one solution\nfor this problem for everyone.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eTo give an example of its usage, imagine a descriptor representing weak\nreferenced values:\u003c/p\u003e\n\u003cpre\u003eimport weakref\n\nclass WeakAttribute:\n def __get__(self, instance, owner):\n return instance.__dict__[self.name]()\n\n def __set__(self, instance, value):\n instance.__dict__[self.name] = weakref.ref(value)\n\n # this is the new initializer:\n def __set_name__(self, owner, name):\n self.name = name\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eSuch a \u003ccode\u003eWeakAttribute\u003c/code\u003e may, for example, be used in a tree structure\nwhere one wants to avoid cyclic references via the parent:\u003c/p\u003e\n\u003cpre\u003eclass TreeNode:\n parent = WeakAttribute()\n\n def __init__(self, parent):\n self.parent = parent\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eNote that the \u003ccode\u003eparent\u003c/code\u003e attribute is used like a normal attribute,\nyet the tree contains no cyclic references and can thus be easily\ngarbage collected when out of use. The \u003ccode\u003eparent\u003c/code\u003e attribute magically\nbecomes \u003ccode\u003eNone\u003c/code\u003e once the parent ceases existing.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eWhile this example looks very trivial, it should be noted that until\nnow such an attribute cannot be defined without the use of a metaclass.\nAnd given that such a metaclass can make life very hard, this kind of\nattribute does not exist yet.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eInitializing descriptors could simply be done in the\n\u003ccode\u003e__init_subclass__\u003c/code\u003e hook. But this would mean that descriptors can\nonly be used in classes that have the proper hook, the generic version\nlike in the example would not work generally. One could also call\n\u003ccode\u003e__set_name__\u003c/code\u003e from within the base implementation of\n\u003ccode\u003eobject.__init_subclass__\u003c/code\u003e. But given that it is a common mistake\nto forget to call \u003ccode\u003esuper()\u003c/code\u003e, it would happen too often that suddenly\ndescriptors are not initialized.\u003c/p\u003e\n\u003ca name=\"user-content-key-benefits\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eKey Benefits\u003c/h2\u003e\u003ca id=\"user-content-key-benefits\" class=\"anchor\" aria-label=\"Permalink: Key Benefits\" href=\"#key-benefits\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003ca name=\"user-content-easier-inheritance-of-definition-time-behaviour\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eEasier inheritance of definition time behaviour\u003c/h3\u003e\u003ca id=\"user-content-easier-inheritance-of-definition-time-behaviour\" class=\"anchor\" aria-label=\"Permalink: Easier inheritance of definition time behaviour\" href=\"#easier-inheritance-of-definition-time-behaviour\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eUnderstanding Python's metaclasses requires a deep understanding of\nthe type system and the class construction process. This is legitimately\nseen as challenging, due to the need to keep multiple moving parts (the code,\nthe metaclass hint, the actual metaclass, the class object, instances of the\nclass object) clearly distinct in your mind. Even when you know the rules,\nit's still easy to make a mistake if you're not being extremely careful.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eUnderstanding the proposed implicit class initialization hook only requires\nordinary method inheritance, which isn't quite as daunting a task. The new\nhook provides a more gradual path towards understanding all of the phases\ninvolved in the class definition process.\u003c/p\u003e\n\u003ca name=\"user-content-reduced-chance-of-metaclass-conflicts\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eReduced chance of metaclass conflicts\u003c/h3\u003e\u003ca id=\"user-content-reduced-chance-of-metaclass-conflicts\" class=\"anchor\" aria-label=\"Permalink: Reduced chance of metaclass conflicts\" href=\"#reduced-chance-of-metaclass-conflicts\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eOne of the big issues that makes library authors reluctant to use metaclasses\n(even when they would be appropriate) is the risk of metaclass conflicts.\nThese occur whenever two unrelated metaclasses are used by the desired\nparents of a class definition. This risk also makes it very difficult to\n\u003cem\u003eadd\u003c/em\u003e a metaclass to a class that has previously been published without one.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eBy contrast, adding an \u003ccode\u003e__init_subclass__\u003c/code\u003e method to an existing type poses\na similar level of risk to adding an \u003ccode\u003e__init__\u003c/code\u003e method: technically, there\nis a risk of breaking poorly implemented subclasses, but when that occurs,\nit is recognised as a bug in the subclass rather than the library author\nbreaching backwards compatibility guarantees.\u003c/p\u003e\n\u003ca name=\"user-content-new-ways-of-using-classes\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eNew Ways of Using Classes\u003c/h2\u003e\u003ca id=\"user-content-new-ways-of-using-classes\" class=\"anchor\" aria-label=\"Permalink: New Ways of Using Classes\" href=\"#new-ways-of-using-classes\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003ca name=\"user-content-subclass-registration\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eSubclass registration\u003c/h3\u003e\u003ca id=\"user-content-subclass-registration\" class=\"anchor\" aria-label=\"Permalink: Subclass registration\" href=\"#subclass-registration\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eEspecially when writing a plugin system, one likes to register new\nsubclasses of a plugin baseclass. This can be done as follows:\u003c/p\u003e\n\u003cpre\u003eclass PluginBase:\n subclasses = []\n\n def __init_subclass__(cls, **kwargs):\n super().__init_subclass__(**kwargs)\n cls.subclasses.append(cls)\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eIn this example, \u003ccode\u003ePluginBase.subclasses\u003c/code\u003e will contain a plain list of all\nsubclasses in the entire inheritance tree. One should note that this also\nworks nicely as a mixin class.\u003c/p\u003e\n\u003ca name=\"user-content-trait-descriptors\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eTrait descriptors\u003c/h3\u003e\u003ca id=\"user-content-trait-descriptors\" class=\"anchor\" aria-label=\"Permalink: Trait descriptors\" href=\"#trait-descriptors\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThere are many designs of Python descriptors in the wild which, for\nexample, check boundaries of values. Often those \"traits\" need some support\nof a metaclass to work. This is how this would look like with this\nPEP:\u003c/p\u003e\n\u003cpre\u003eclass Trait:\n def __init__(self, minimum, maximum):\n self.minimum = minimum\n self.maximum = maximum\n\n def __get__(self, instance, owner):\n return instance.__dict__[self.key]\n\n def __set__(self, instance, value):\n if self.minimum \u0026lt; value \u0026lt; self.maximum:\n instance.__dict__[self.key] = value\n else:\n raise ValueError(\"value not in range\")\n\n def __set_name__(self, owner, name):\n self.key = name\n\u003c/pre\u003e\n\u003ca name=\"user-content-implementation-details\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eImplementation Details\u003c/h2\u003e\u003ca id=\"user-content-implementation-details\" class=\"anchor\" aria-label=\"Permalink: Implementation Details\" href=\"#implementation-details\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe hooks are called in the following order: \u003ccode\u003etype.__new__\u003c/code\u003e calls\nthe \u003ccode\u003e__set_name__\u003c/code\u003e hooks on the descriptor after the new class has been\ninitialized. Then it calls \u003ccode\u003e__init_subclass__\u003c/code\u003e on the base class, on\n\u003ccode\u003esuper()\u003c/code\u003e, to be precise. This means that subclass initializers already\nsee the fully initialized descriptors. This way, \u003ccode\u003e__init_subclass__\u003c/code\u003e users\ncan fix all descriptors again if this is needed.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAnother option would have been to call \u003ccode\u003e__set_name__\u003c/code\u003e in the base\nimplementation of \u003ccode\u003eobject.__init_subclass__\u003c/code\u003e. This way it would be possible\neven to prevent \u003ccode\u003e__set_name__\u003c/code\u003e from being called. Most of the times,\nhowever, such a prevention would be accidental, as it often happens that a call\nto \u003ccode\u003esuper()\u003c/code\u003e is forgotten.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAs a third option, all the work could have been done in \u003ccode\u003etype.__init__\u003c/code\u003e.\nMost metaclasses do their work in \u003ccode\u003e__new__\u003c/code\u003e, as this is recommended by\nthe documentation. Many metaclasses modify their arguments before they\npass them over to \u003ccode\u003esuper().__new__\u003c/code\u003e. For compatibility with those kind\nof classes, the hooks should be called from \u003ccode\u003e__new__\u003c/code\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAnother small change should be done: in the current implementation of\nCPython, \u003ccode\u003etype.__init__\u003c/code\u003e explicitly forbids the use of keyword arguments,\nwhile \u003ccode\u003etype.__new__\u003c/code\u003e allows for its attributes to be shipped as keyword\narguments. This is weirdly incoherent, and thus it should be forbidden.\nWhile it would be possible to retain the current behavior, it would be better\nif this was fixed, as it is probably not used at all: the only use case would\nbe that at metaclass calls its \u003ccode\u003esuper().__new__\u003c/code\u003e with \u003cem\u003ename\u003c/em\u003e, \u003cem\u003ebases\u003c/em\u003e and\n\u003cem\u003edict\u003c/em\u003e (yes, \u003cem\u003edict\u003c/em\u003e, not \u003cem\u003enamespace\u003c/em\u003e or \u003cem\u003ens\u003c/em\u003e as mostly used with modern\nmetaclasses) as keyword arguments. This should not be done. This little\nchange simplifies the implementation of this PEP significantly, while\nimproving the coherence of Python overall.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAs a second change, the new \u003ccode\u003etype.__init__\u003c/code\u003e just ignores keyword\narguments. Currently, it insists that no keyword arguments are given. This\nleads to a (wanted) error if one gives keyword arguments to a class declaration\nif the metaclass does not process them. Metaclass authors that do want to\naccept keyword arguments must filter them out by overriding \u003ccode\u003e__init__\u003c/code\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eIn the new code, it is not \u003ccode\u003e__init__\u003c/code\u003e that complains about keyword arguments,\nbut \u003ccode\u003e__init_subclass__\u003c/code\u003e, whose default implementation takes no arguments. In\na classical inheritance scheme using the method resolution order, each\n\u003ccode\u003e__init_subclass__\u003c/code\u003e may take out it's keyword arguments until none are left,\nwhich is checked by the default implementation of \u003ccode\u003e__init_subclass__\u003c/code\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor readers who prefer reading Python over English, this PEP proposes to\nreplace the current \u003ccode\u003etype\u003c/code\u003e and \u003ccode\u003eobject\u003c/code\u003e with the following:\u003c/p\u003e\n\u003cpre\u003eclass NewType(type):\n def __new__(cls, *args, **kwargs):\n if len(args) != 3:\n return super().__new__(cls, *args)\n name, bases, ns = args\n init = ns.get('__init_subclass__')\n if isinstance(init, types.FunctionType):\n ns['__init_subclass__'] = classmethod(init)\n self = super().__new__(cls, name, bases, ns)\n for k, v in self.__dict__.items():\n func = getattr(v, '__set_name__', None)\n if func is not None:\n func(self, k)\n super(self, self).__init_subclass__(**kwargs)\n return self\n\n def __init__(self, name, bases, ns, **kwargs):\n super().__init__(name, bases, ns)\n\nclass NewObject(object):\n @classmethod\n def __init_subclass__(cls):\n pass\n\u003c/pre\u003e\n\u003ca name=\"user-content-reference-implementation\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eReference Implementation\u003c/h2\u003e\u003ca id=\"user-content-reference-implementation\" class=\"anchor\" aria-label=\"Permalink: Reference Implementation\" href=\"#reference-implementation\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe reference implementation for this PEP is attached to\n\u003ca href=\"http://bugs.python.org/issue27366\" rel=\"nofollow\"\u003eissue 27366\u003c/a\u003e.\u003c/p\u003e\n\u003ca name=\"user-content-backward-compatibility-issues\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eBackward compatibility issues\u003c/h2\u003e\u003ca id=\"user-content-backward-compatibility-issues\" class=\"anchor\" aria-label=\"Permalink: Backward compatibility issues\" href=\"#backward-compatibility-issues\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe exact calling sequence in \u003ccode\u003etype.__new__\u003c/code\u003e is slightly changed, raising\nfears of backwards compatibility. It should be assured by tests that common use\ncases behave as desired.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe following class definitions (except the one defining the metaclass)\ncontinue to fail with a \u003ccode\u003eTypeError\u003c/code\u003e as superfluous class arguments are passed:\u003c/p\u003e\n\u003cpre\u003eclass MyMeta(type):\n pass\n\nclass MyClass(metaclass=MyMeta, otherarg=1):\n pass\n\nMyMeta(\"MyClass\", (), otherargs=1)\n\nimport types\ntypes.new_class(\"MyClass\", (), dict(metaclass=MyMeta, otherarg=1))\ntypes.prepare_class(\"MyClass\", (), dict(metaclass=MyMeta, otherarg=1))\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eA metaclass defining only a \u003ccode\u003e__new__\u003c/code\u003e method which is interested in keyword\narguments now does not need to define an \u003ccode\u003e__init__\u003c/code\u003e method anymore, as the\ndefault \u003ccode\u003etype.__init__\u003c/code\u003e ignores keyword arguments. This is nicely in line\nwith the recommendation to override \u003ccode\u003e__new__\u003c/code\u003e in metaclasses instead of\n\u003ccode\u003e__init__\u003c/code\u003e. The following code does not fail anymore:\u003c/p\u003e\n\u003cpre\u003eclass MyMeta(type):\n def __new__(cls, name, bases, namespace, otherarg):\n return super().__new__(cls, name, bases, namespace)\n\nclass MyClass(metaclass=MyMeta, otherarg=1):\n pass\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eOnly defining an \u003ccode\u003e__init__\u003c/code\u003e method in a metaclass continues to fail with\n\u003ccode\u003eTypeError\u003c/code\u003e if keyword arguments are given:\u003c/p\u003e\n\u003cpre\u003eclass MyMeta(type):\n def __init__(self, name, bases, namespace, otherarg):\n super().__init__(name, bases, namespace)\n\nclass MyClass(metaclass=MyMeta, otherarg=1):\n pass\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eDefining both \u003ccode\u003e__init__\u003c/code\u003e and \u003ccode\u003e__new__\u003c/code\u003e continues to work fine.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAbout the only thing that stops working is passing the arguments of\n\u003ccode\u003etype.__new__\u003c/code\u003e as keyword arguments:\u003c/p\u003e\n\u003cpre\u003eclass MyMeta(type):\n def __new__(cls, name, bases, namespace):\n return super().__new__(cls, name=name, bases=bases,\n dict=namespace)\n\nclass MyClass(metaclass=MyMeta):\n pass\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eThis will now raise \u003ccode\u003eTypeError\u003c/code\u003e, but this is weird code, and easy\nto fix even if someone used this feature.\u003c/p\u003e\n\u003ca name=\"user-content-rejected-design-options\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eRejected Design Options\u003c/h2\u003e\u003ca id=\"user-content-rejected-design-options\" class=\"anchor\" aria-label=\"Permalink: Rejected Design Options\" href=\"#rejected-design-options\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003ca name=\"user-content-calling-the-hook-on-the-class-itself\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eCalling the hook on the class itself\u003c/h3\u003e\u003ca id=\"user-content-calling-the-hook-on-the-class-itself\" class=\"anchor\" aria-label=\"Permalink: Calling the hook on the class itself\" href=\"#calling-the-hook-on-the-class-itself\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAdding an \u003ccode\u003e__autodecorate__\u003c/code\u003e hook that would be called on the class\nitself was the proposed idea of \u003ca href=\"http://www.python.org/dev/peps/pep-0422\" rel=\"nofollow\"\u003ePEP 422\u003c/a\u003e. Most examples work the same\nway or even better if the hook is called only on strict subclasses. In general,\nit is much easier to arrange to explicitly call the hook on the class in which it\nis defined (to opt-in to such a behavior) than to opt-out (by remember to check for\n\u003ccode\u003ecls is __class\u003c/code\u003e in the hook body), meaning that one does not want the hook to be\ncalled on the class it is defined in.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThis becomes most evident if the class in question is designed as a\nmixin: it is very unlikely that the code of the mixin is to be\nexecuted for the mixin class itself, as it is not supposed to be a\ncomplete class on its own.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe original proposal also made major changes in the class\ninitialization process, rendering it impossible to back-port the\nproposal to older Python versions.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eWhen it's desired to also call the hook on the base class, two mechanisms are available:\u003c/p\u003e\n\u003col dir=\"auto\"\u003e\n\u003cli\u003eIntroduce an additional mixin class just to hold the \u003ccode\u003e__init_subclass__\u003c/code\u003e\nimplementation. The original \"base\" class can then list the new mixin as its\nfirst parent class.\u003c/li\u003e\n\u003cli\u003eImplement the desired behaviour as an independent class decorator, and apply that\ndecorator explicitly to the base class, and then implicitly to subclasses via\n\u003ccode\u003e__init_subclass__\u003c/code\u003e.\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp dir=\"auto\"\u003eCalling \u003ccode\u003e__init_subclass__\u003c/code\u003e explicitly from a class decorator will generally be\nundesirable, as this will also typically call \u003ccode\u003e__init_subclass__\u003c/code\u003e a second time on\nthe parent class, which is unlikely to be desired behaviour.\u003c/p\u003e\n\u003ca name=\"user-content-other-variants-of-calling-the-hooks\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eOther variants of calling the hooks\u003c/h3\u003e\u003ca id=\"user-content-other-variants-of-calling-the-hooks\" class=\"anchor\" aria-label=\"Permalink: Other variants of calling the hooks\" href=\"#other-variants-of-calling-the-hooks\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eOther names for the hook were presented, namely \u003ccode\u003e__decorate__\u003c/code\u003e or\n\u003ccode\u003e__autodecorate__\u003c/code\u003e. This proposal opts for \u003ccode\u003e__init_subclass__\u003c/code\u003e as\nit is very close to the \u003ccode\u003e__init__\u003c/code\u003e method, just for the subclass,\nwhile it is not very close to decorators, as it does not return the\nclass.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor the \u003ccode\u003e__set_name__\u003c/code\u003e hook other names have been proposed as well,\n\u003ccode\u003e__set_owner__\u003c/code\u003e, \u003ccode\u003e__set_ownership__\u003c/code\u003e and \u003ccode\u003e__init_descriptor__\u003c/code\u003e.\u003c/p\u003e\n\u003ca name=\"user-content-requiring-an-explicit-decorator-on-init-subclass\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eRequiring an explicit decorator on \u003ccode\u003e__init_subclass__\u003c/code\u003e\u003c/h3\u003e\u003ca id=\"user-content-requiring-an-explicit-decorator-on-__init_subclass__\" class=\"anchor\" aria-label=\"Permalink: Requiring an explicit decorator on __init_subclass__\" href=\"#requiring-an-explicit-decorator-on-__init_subclass__\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eOne could require the explicit use of \u003ccode\u003e@classmethod\u003c/code\u003e on the\n\u003ccode\u003e__init_subclass__\u003c/code\u003e decorator. It was made implicit since there's no\nsensible interpretation for leaving it out, and that case would need\nto be detected anyway in order to give a useful error message.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThis decision was reinforced after noticing that the user experience of\ndefining \u003ccode\u003e__prepare__\u003c/code\u003e and forgetting the \u003ccode\u003e@classmethod\u003c/code\u003e method\ndecorator is singularly incomprehensible (particularly since \u003ca href=\"http://www.python.org/dev/peps/pep-3115\" rel=\"nofollow\"\u003ePEP 3115\u003c/a\u003e\ndocuments it as an ordinary method, and the current documentation doesn't\nexplicitly say anything one way or the other).\u003c/p\u003e\n\u003ca name=\"user-content-a-more-new-like-hook\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eA more \u003ccode\u003e__new__\u003c/code\u003e-like hook\u003c/h3\u003e\u003ca id=\"user-content-a-more-__new__-like-hook\" class=\"anchor\" aria-label=\"Permalink: A more __new__-like hook\" href=\"#a-more-__new__-like-hook\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIn \u003ca href=\"http://www.python.org/dev/peps/pep-0422\" rel=\"nofollow\"\u003ePEP 422\u003c/a\u003e the hook worked more like the \u003ccode\u003e__new__\u003c/code\u003e method than the\n\u003ccode\u003e__init__\u003c/code\u003e method, meaning that it returned a class instead of\nmodifying one. This allows a bit more flexibility, but at the cost\nof much harder implementation and undesired side effects.\u003c/p\u003e\n\u003ca name=\"user-content-adding-a-class-attribute-with-the-attribute-order\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eAdding a class attribute with the attribute order\u003c/h3\u003e\u003ca id=\"user-content-adding-a-class-attribute-with-the-attribute-order\" class=\"anchor\" aria-label=\"Permalink: Adding a class attribute with the attribute order\" href=\"#adding-a-class-attribute-with-the-attribute-order\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis got its own \u003ca href=\"http://www.python.org/dev/peps/pep-0520\" rel=\"nofollow\"\u003ePEP 520\u003c/a\u003e.\u003c/p\u003e\n\u003ca name=\"user-content-history\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eHistory\u003c/h2\u003e\u003ca id=\"user-content-history\" class=\"anchor\" aria-label=\"Permalink: History\" href=\"#history\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis used to be a competing proposal to \u003ca href=\"http://www.python.org/dev/peps/pep-0422\" rel=\"nofollow\"\u003ePEP 422\u003c/a\u003e by Alyssa Coghlan and Daniel\nUrban. \u003ca href=\"http://www.python.org/dev/peps/pep-0422\" rel=\"nofollow\"\u003ePEP 422\u003c/a\u003e intended to achieve the same goals as this PEP, but with a\ndifferent way of implementation. In the meantime, \u003ca href=\"http://www.python.org/dev/peps/pep-0422\" rel=\"nofollow\"\u003ePEP 422\u003c/a\u003e has been withdrawn\nfavouring this approach.\u003c/p\u003e\n\u003ca name=\"user-content-references\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eReferences\u003c/h2\u003e\u003ca id=\"user-content-references\" class=\"anchor\" aria-label=\"Permalink: References\" href=\"#references\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003ca name=\"user-content-copyright\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eCopyright\u003c/h2\u003e\u003ca id=\"user-content-copyright\" class=\"anchor\" aria-label=\"Permalink: Copyright\" href=\"#copyright\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis document has been placed in the public domain.\u003c/p\u003e\n\n\u003c/article\u003e","renderedFileInfo":null,"shortPath":null,"symbolsEnabled":true,"tabSize":8,"topBannersInfo":{"overridingGlobalFundingFile":false,"globalPreferredFundingPath":"/python/.github/blob/5a0a7cb55767fb3667fe2391bcb66bda638edcb4/FUNDING.yml","showInvalidCitationWarning":false,"citationHelpUrl":"https://docs.github.com/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-citation-files","actionsOnboardingTip":null},"truncated":false,"viewable":true,"workflowRedirectUrl":null,"symbols":{"timed_out":false,"not_analyzed":true,"symbols":[]}},"copilotInfo":null,"copilotAccessAllowed":false,"modelsAccessAllowed":false,"modelsRepoIntegrationEnabled":false,"csrf_tokens":{"/python/peps/branches":{"post":"okPSRAeflivv9eWEWuRuWrEnKrlPL6yUFXNsYU3Gen1KzcMb7Ngb7xHxwx1zpg5oglJkb7nAwZdYwAdFdcjS7A"},"/repos/preferences":{"post":"oERBtpFnMi21u6SxPL9oKpG_stKrKVBKn7RjrC8gwJv4mPTbytGEY9ryw8_bGshwUAARhQO0URj4tTZnBXwO7A"}}},"title":"peps/peps/pep-0487.rst 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3Z"></path></svg></button></div></div></div></div><div></div></div><div class="Box-sc-g0xbh4-0 hGyMdv"><section aria-labelledby="file-name-id-wide file-name-id-mobile" class="Box-sc-g0xbh4-0 fGqKFv"><div class="Box-sc-g0xbh4-0 eoaCFS js-snippet-clipboard-copy-unpositioned undefined" data-hpc="true"><article class="markdown-body entry-content container-lg" itemprop="text"><p dir="auto">PEP: 487 Title: Simpler customisation of class creation Author: Martin Teichmann <<a href="mailto:lkb.teichmann@gmail.com">lkb.teichmann@gmail.com</a>>, Status: Final Type: Standards Track Created: 27-Feb-2015 Python-Version: 3.6 Post-History: 27-Feb-2015, 05-Feb-2016, 24-Jun-2016, 02-Jul-2016, 13-Jul-2016 Replaces: 422 Resolution: <a href="https://mail.python.org/pipermail/python-dev/2016-July/145629.html" rel="nofollow">https://mail.python.org/pipermail/python-dev/2016-July/145629.html</a></p> <a name="user-content-abstract"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Abstract</h2><a id="user-content-abstract" class="anchor" aria-label="Permalink: Abstract" href="#abstract"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Currently, customising class creation requires the use of a custom metaclass. This custom metaclass then persists for the entire lifecycle of the class, creating the potential for spurious metaclass conflicts.</p> <p dir="auto">This PEP proposes to instead support a wide range of customisation scenarios through a new <code>__init_subclass__</code> hook in the class body, and a hook to initialize attributes.</p> <p dir="auto">The new mechanism should be easier to understand and use than implementing a custom metaclass, and thus should provide a gentler introduction to the full power of Python's metaclass machinery.</p> <a name="user-content-background"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Background</h2><a id="user-content-background" class="anchor" aria-label="Permalink: Background" href="#background"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Metaclasses are a powerful tool to customize class creation. They have, however, the problem that there is no automatic way to combine metaclasses. If one wants to use two metaclasses for a class, a new metaclass combining those two needs to be created, typically manually.</p> <p dir="auto">This need often occurs as a surprise to a user: inheriting from two base classes coming from two different libraries suddenly raises the necessity to manually create a combined metaclass, where typically one is not interested in those details about the libraries at all. This becomes even worse if one library starts to make use of a metaclass which it has not done before. While the library itself continues to work perfectly, suddenly every code combining those classes with classes from another library fails.</p> <a name="user-content-proposal"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Proposal</h2><a id="user-content-proposal" class="anchor" aria-label="Permalink: Proposal" href="#proposal"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">While there are many possible ways to use a metaclass, the vast majority of use cases falls into just three categories: some initialization code running after class creation, the initialization of descriptors and keeping the order in which class attributes were defined.</p> <p dir="auto">The first two categories can easily be achieved by having simple hooks into the class creation:</p> <ol dir="auto"> <li>An <code>__init_subclass__</code> hook that initializes all subclasses of a given class.</li> <li>upon class creation, a <code>__set_name__</code> hook is called on all the attribute (descriptors) defined in the class, and</li> </ol> <p dir="auto">The third category is the topic of another PEP, <a href="http://www.python.org/dev/peps/pep-0520" rel="nofollow">PEP 520</a>.</p> <p dir="auto">As an example, the first use case looks as follows:</p> <pre>>>> class QuestBase: ... # this is implicitly a @classmethod (see below for motivation) ... def __init_subclass__(cls, swallow, **kwargs): ... cls.swallow = swallow ... super().__init_subclass__(**kwargs) >>> class Quest(QuestBase, swallow="african"): ... pass >>> Quest.swallow 'african' </pre> <p dir="auto">The base class <code>object</code> contains an empty <code>__init_subclass__</code> method which serves as an endpoint for cooperative multiple inheritance. Note that this method has no keyword arguments, meaning that all methods which are more specialized have to process all keyword arguments.</p> <p dir="auto">This general proposal is not a new idea (it was first suggested for inclusion in the language definition <a href="https://mail.python.org/pipermail/python-dev/2001-November/018651.html" rel="nofollow">more than 10 years ago</a>, and a similar mechanism has long been supported by <a href="http://docs.zope.org/zope_secrets/extensionclass.html" rel="nofollow">Zope's ExtensionClass</a>), but the situation has changed sufficiently in recent years that the idea is worth reconsidering for inclusion.</p> <p dir="auto">The second part of the proposal adds an <code>__set_name__</code> initializer for class attributes, especially if they are descriptors. Descriptors are defined in the body of a class, but they do not know anything about that class, they do not even know the name they are accessed with. They do get to know their owner once <code>__get__</code> is called, but still they do not know their name. This is unfortunate, for example they cannot put their associated value into their object's <code>__dict__</code> under their name, since they do not know that name. This problem has been solved many times, and is one of the most important reasons to have a metaclass in a library. While it would be easy to implement such a mechanism using the first part of the proposal, it makes sense to have one solution for this problem for everyone.</p> <p dir="auto">To give an example of its usage, imagine a descriptor representing weak referenced values:</p> <pre>import weakref class WeakAttribute: def __get__(self, instance, owner): return instance.__dict__[self.name]() def __set__(self, instance, value): instance.__dict__[self.name] = weakref.ref(value) # this is the new initializer: def __set_name__(self, owner, name): self.name = name </pre> <p dir="auto">Such a <code>WeakAttribute</code> may, for example, be used in a tree structure where one wants to avoid cyclic references via the parent:</p> <pre>class TreeNode: parent = WeakAttribute() def __init__(self, parent): self.parent = parent </pre> <p dir="auto">Note that the <code>parent</code> attribute is used like a normal attribute, yet the tree contains no cyclic references and can thus be easily garbage collected when out of use. The <code>parent</code> attribute magically becomes <code>None</code> once the parent ceases existing.</p> <p dir="auto">While this example looks very trivial, it should be noted that until now such an attribute cannot be defined without the use of a metaclass. And given that such a metaclass can make life very hard, this kind of attribute does not exist yet.</p> <p dir="auto">Initializing descriptors could simply be done in the <code>__init_subclass__</code> hook. But this would mean that descriptors can only be used in classes that have the proper hook, the generic version like in the example would not work generally. One could also call <code>__set_name__</code> from within the base implementation of <code>object.__init_subclass__</code>. But given that it is a common mistake to forget to call <code>super()</code>, it would happen too often that suddenly descriptors are not initialized.</p> <a name="user-content-key-benefits"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Key Benefits</h2><a id="user-content-key-benefits" class="anchor" aria-label="Permalink: Key Benefits" href="#key-benefits"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <a name="user-content-easier-inheritance-of-definition-time-behaviour"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Easier inheritance of definition time behaviour</h3><a id="user-content-easier-inheritance-of-definition-time-behaviour" class="anchor" aria-label="Permalink: Easier inheritance of definition time behaviour" href="#easier-inheritance-of-definition-time-behaviour"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Understanding Python's metaclasses requires a deep understanding of the type system and the class construction process. This is legitimately seen as challenging, due to the need to keep multiple moving parts (the code, the metaclass hint, the actual metaclass, the class object, instances of the class object) clearly distinct in your mind. Even when you know the rules, it's still easy to make a mistake if you're not being extremely careful.</p> <p dir="auto">Understanding the proposed implicit class initialization hook only requires ordinary method inheritance, which isn't quite as daunting a task. The new hook provides a more gradual path towards understanding all of the phases involved in the class definition process.</p> <a name="user-content-reduced-chance-of-metaclass-conflicts"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Reduced chance of metaclass conflicts</h3><a id="user-content-reduced-chance-of-metaclass-conflicts" class="anchor" aria-label="Permalink: Reduced chance of metaclass conflicts" href="#reduced-chance-of-metaclass-conflicts"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">One of the big issues that makes library authors reluctant to use metaclasses (even when they would be appropriate) is the risk of metaclass conflicts. These occur whenever two unrelated metaclasses are used by the desired parents of a class definition. This risk also makes it very difficult to <em>add</em> a metaclass to a class that has previously been published without one.</p> <p dir="auto">By contrast, adding an <code>__init_subclass__</code> method to an existing type poses a similar level of risk to adding an <code>__init__</code> method: technically, there is a risk of breaking poorly implemented subclasses, but when that occurs, it is recognised as a bug in the subclass rather than the library author breaching backwards compatibility guarantees.</p> <a name="user-content-new-ways-of-using-classes"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">New Ways of Using Classes</h2><a id="user-content-new-ways-of-using-classes" class="anchor" aria-label="Permalink: New Ways of Using Classes" href="#new-ways-of-using-classes"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <a name="user-content-subclass-registration"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Subclass registration</h3><a id="user-content-subclass-registration" class="anchor" aria-label="Permalink: Subclass registration" href="#subclass-registration"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Especially when writing a plugin system, one likes to register new subclasses of a plugin baseclass. This can be done as follows:</p> <pre>class PluginBase: subclasses = [] def __init_subclass__(cls, **kwargs): super().__init_subclass__(**kwargs) cls.subclasses.append(cls) </pre> <p dir="auto">In this example, <code>PluginBase.subclasses</code> will contain a plain list of all subclasses in the entire inheritance tree. One should note that this also works nicely as a mixin class.</p> <a name="user-content-trait-descriptors"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Trait descriptors</h3><a id="user-content-trait-descriptors" class="anchor" aria-label="Permalink: Trait descriptors" href="#trait-descriptors"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">There are many designs of Python descriptors in the wild which, for example, check boundaries of values. Often those "traits" need some support of a metaclass to work. This is how this would look like with this PEP:</p> <pre>class Trait: def __init__(self, minimum, maximum): self.minimum = minimum self.maximum = maximum def __get__(self, instance, owner): return instance.__dict__[self.key] def __set__(self, instance, value): if self.minimum < value < self.maximum: instance.__dict__[self.key] = value else: raise ValueError("value not in range") def __set_name__(self, owner, name): self.key = name </pre> <a name="user-content-implementation-details"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Implementation Details</h2><a id="user-content-implementation-details" class="anchor" aria-label="Permalink: Implementation Details" href="#implementation-details"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The hooks are called in the following order: <code>type.__new__</code> calls the <code>__set_name__</code> hooks on the descriptor after the new class has been initialized. Then it calls <code>__init_subclass__</code> on the base class, on <code>super()</code>, to be precise. This means that subclass initializers already see the fully initialized descriptors. This way, <code>__init_subclass__</code> users can fix all descriptors again if this is needed.</p> <p dir="auto">Another option would have been to call <code>__set_name__</code> in the base implementation of <code>object.__init_subclass__</code>. This way it would be possible even to prevent <code>__set_name__</code> from being called. Most of the times, however, such a prevention would be accidental, as it often happens that a call to <code>super()</code> is forgotten.</p> <p dir="auto">As a third option, all the work could have been done in <code>type.__init__</code>. Most metaclasses do their work in <code>__new__</code>, as this is recommended by the documentation. Many metaclasses modify their arguments before they pass them over to <code>super().__new__</code>. For compatibility with those kind of classes, the hooks should be called from <code>__new__</code>.</p> <p dir="auto">Another small change should be done: in the current implementation of CPython, <code>type.__init__</code> explicitly forbids the use of keyword arguments, while <code>type.__new__</code> allows for its attributes to be shipped as keyword arguments. This is weirdly incoherent, and thus it should be forbidden. While it would be possible to retain the current behavior, it would be better if this was fixed, as it is probably not used at all: the only use case would be that at metaclass calls its <code>super().__new__</code> with <em>name</em>, <em>bases</em> and <em>dict</em> (yes, <em>dict</em>, not <em>namespace</em> or <em>ns</em> as mostly used with modern metaclasses) as keyword arguments. This should not be done. This little change simplifies the implementation of this PEP significantly, while improving the coherence of Python overall.</p> <p dir="auto">As a second change, the new <code>type.__init__</code> just ignores keyword arguments. Currently, it insists that no keyword arguments are given. This leads to a (wanted) error if one gives keyword arguments to a class declaration if the metaclass does not process them. Metaclass authors that do want to accept keyword arguments must filter them out by overriding <code>__init__</code>.</p> <p dir="auto">In the new code, it is not <code>__init__</code> that complains about keyword arguments, but <code>__init_subclass__</code>, whose default implementation takes no arguments. In a classical inheritance scheme using the method resolution order, each <code>__init_subclass__</code> may take out it's keyword arguments until none are left, which is checked by the default implementation of <code>__init_subclass__</code>.</p> <p dir="auto">For readers who prefer reading Python over English, this PEP proposes to replace the current <code>type</code> and <code>object</code> with the following:</p> <pre>class NewType(type): def __new__(cls, *args, **kwargs): if len(args) != 3: return super().__new__(cls, *args) name, bases, ns = args init = ns.get('__init_subclass__') if isinstance(init, types.FunctionType): ns['__init_subclass__'] = classmethod(init) self = super().__new__(cls, name, bases, ns) for k, v in self.__dict__.items(): func = getattr(v, '__set_name__', None) if func is not None: func(self, k) super(self, self).__init_subclass__(**kwargs) return self def __init__(self, name, bases, ns, **kwargs): super().__init__(name, bases, ns) class NewObject(object): @classmethod def __init_subclass__(cls): pass </pre> <a name="user-content-reference-implementation"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Reference Implementation</h2><a id="user-content-reference-implementation" class="anchor" aria-label="Permalink: Reference Implementation" href="#reference-implementation"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The reference implementation for this PEP is attached to <a href="http://bugs.python.org/issue27366" rel="nofollow">issue 27366</a>.</p> <a name="user-content-backward-compatibility-issues"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Backward compatibility issues</h2><a id="user-content-backward-compatibility-issues" class="anchor" aria-label="Permalink: Backward compatibility issues" href="#backward-compatibility-issues"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The exact calling sequence in <code>type.__new__</code> is slightly changed, raising fears of backwards compatibility. It should be assured by tests that common use cases behave as desired.</p> <p dir="auto">The following class definitions (except the one defining the metaclass) continue to fail with a <code>TypeError</code> as superfluous class arguments are passed:</p> <pre>class MyMeta(type): pass class MyClass(metaclass=MyMeta, otherarg=1): pass MyMeta("MyClass", (), otherargs=1) import types types.new_class("MyClass", (), dict(metaclass=MyMeta, otherarg=1)) types.prepare_class("MyClass", (), dict(metaclass=MyMeta, otherarg=1)) </pre> <p dir="auto">A metaclass defining only a <code>__new__</code> method which is interested in keyword arguments now does not need to define an <code>__init__</code> method anymore, as the default <code>type.__init__</code> ignores keyword arguments. This is nicely in line with the recommendation to override <code>__new__</code> in metaclasses instead of <code>__init__</code>. The following code does not fail anymore:</p> <pre>class MyMeta(type): def __new__(cls, name, bases, namespace, otherarg): return super().__new__(cls, name, bases, namespace) class MyClass(metaclass=MyMeta, otherarg=1): pass </pre> <p dir="auto">Only defining an <code>__init__</code> method in a metaclass continues to fail with <code>TypeError</code> if keyword arguments are given:</p> <pre>class MyMeta(type): def __init__(self, name, bases, namespace, otherarg): super().__init__(name, bases, namespace) class MyClass(metaclass=MyMeta, otherarg=1): pass </pre> <p dir="auto">Defining both <code>__init__</code> and <code>__new__</code> continues to work fine.</p> <p dir="auto">About the only thing that stops working is passing the arguments of <code>type.__new__</code> as keyword arguments:</p> <pre>class MyMeta(type): def __new__(cls, name, bases, namespace): return super().__new__(cls, name=name, bases=bases, dict=namespace) class MyClass(metaclass=MyMeta): pass </pre> <p dir="auto">This will now raise <code>TypeError</code>, but this is weird code, and easy to fix even if someone used this feature.</p> <a name="user-content-rejected-design-options"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Rejected Design Options</h2><a id="user-content-rejected-design-options" class="anchor" aria-label="Permalink: Rejected Design Options" href="#rejected-design-options"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <a name="user-content-calling-the-hook-on-the-class-itself"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Calling the hook on the class itself</h3><a id="user-content-calling-the-hook-on-the-class-itself" class="anchor" aria-label="Permalink: Calling the hook on the class itself" href="#calling-the-hook-on-the-class-itself"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Adding an <code>__autodecorate__</code> hook that would be called on the class itself was the proposed idea of <a href="http://www.python.org/dev/peps/pep-0422" rel="nofollow">PEP 422</a>. Most examples work the same way or even better if the hook is called only on strict subclasses. In general, it is much easier to arrange to explicitly call the hook on the class in which it is defined (to opt-in to such a behavior) than to opt-out (by remember to check for <code>cls is __class</code> in the hook body), meaning that one does not want the hook to be called on the class it is defined in.</p> <p dir="auto">This becomes most evident if the class in question is designed as a mixin: it is very unlikely that the code of the mixin is to be executed for the mixin class itself, as it is not supposed to be a complete class on its own.</p> <p dir="auto">The original proposal also made major changes in the class initialization process, rendering it impossible to back-port the proposal to older Python versions.</p> <p dir="auto">When it's desired to also call the hook on the base class, two mechanisms are available:</p> <ol dir="auto"> <li>Introduce an additional mixin class just to hold the <code>__init_subclass__</code> implementation. The original "base" class can then list the new mixin as its first parent class.</li> <li>Implement the desired behaviour as an independent class decorator, and apply that decorator explicitly to the base class, and then implicitly to subclasses via <code>__init_subclass__</code>.</li> </ol> <p dir="auto">Calling <code>__init_subclass__</code> explicitly from a class decorator will generally be undesirable, as this will also typically call <code>__init_subclass__</code> a second time on the parent class, which is unlikely to be desired behaviour.</p> <a name="user-content-other-variants-of-calling-the-hooks"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Other variants of calling the hooks</h3><a id="user-content-other-variants-of-calling-the-hooks" class="anchor" aria-label="Permalink: Other variants of calling the hooks" href="#other-variants-of-calling-the-hooks"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Other names for the hook were presented, namely <code>__decorate__</code> or <code>__autodecorate__</code>. This proposal opts for <code>__init_subclass__</code> as it is very close to the <code>__init__</code> method, just for the subclass, while it is not very close to decorators, as it does not return the class.</p> <p dir="auto">For the <code>__set_name__</code> hook other names have been proposed as well, <code>__set_owner__</code>, <code>__set_ownership__</code> and <code>__init_descriptor__</code>.</p> <a name="user-content-requiring-an-explicit-decorator-on-init-subclass"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Requiring an explicit decorator on <code>__init_subclass__</code></h3><a id="user-content-requiring-an-explicit-decorator-on-__init_subclass__" class="anchor" aria-label="Permalink: Requiring an explicit decorator on __init_subclass__" href="#requiring-an-explicit-decorator-on-__init_subclass__"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">One could require the explicit use of <code>@classmethod</code> on the <code>__init_subclass__</code> decorator. It was made implicit since there's no sensible interpretation for leaving it out, and that case would need to be detected anyway in order to give a useful error message.</p> <p dir="auto">This decision was reinforced after noticing that the user experience of defining <code>__prepare__</code> and forgetting the <code>@classmethod</code> method decorator is singularly incomprehensible (particularly since <a href="http://www.python.org/dev/peps/pep-3115" rel="nofollow">PEP 3115</a> documents it as an ordinary method, and the current documentation doesn't explicitly say anything one way or the other).</p> <a name="user-content-a-more-new-like-hook"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">A more <code>__new__</code>-like hook</h3><a id="user-content-a-more-__new__-like-hook" class="anchor" aria-label="Permalink: A more __new__-like hook" href="#a-more-__new__-like-hook"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">In <a href="http://www.python.org/dev/peps/pep-0422" rel="nofollow">PEP 422</a> the hook worked more like the <code>__new__</code> method than the <code>__init__</code> method, meaning that it returned a class instead of modifying one. This allows a bit more flexibility, but at the cost of much harder implementation and undesired side effects.</p> <a name="user-content-adding-a-class-attribute-with-the-attribute-order"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Adding a class attribute with the attribute order</h3><a id="user-content-adding-a-class-attribute-with-the-attribute-order" class="anchor" aria-label="Permalink: Adding a class attribute with the attribute order" href="#adding-a-class-attribute-with-the-attribute-order"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This got its own <a href="http://www.python.org/dev/peps/pep-0520" rel="nofollow">PEP 520</a>.</p> <a name="user-content-history"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">History</h2><a id="user-content-history" class="anchor" aria-label="Permalink: History" href="#history"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This used to be a competing proposal to <a href="http://www.python.org/dev/peps/pep-0422" rel="nofollow">PEP 422</a> by Alyssa Coghlan and Daniel Urban. <a href="http://www.python.org/dev/peps/pep-0422" rel="nofollow">PEP 422</a> intended to achieve the same goals as this PEP, but with a different way of implementation. In the meantime, <a href="http://www.python.org/dev/peps/pep-0422" rel="nofollow">PEP 422</a> has been withdrawn favouring this approach.</p> <a name="user-content-references"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">References</h2><a id="user-content-references" class="anchor" aria-label="Permalink: References" href="#references"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <a name="user-content-copyright"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Copyright</h2><a id="user-content-copyright" class="anchor" aria-label="Permalink: Copyright" href="#copyright"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This document 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</svg> </button> <div class="octocat-spinner my-6 js-details-dialog-spinner"></div> </details-dialog> </details> </template> <div class="Popover js-hovercard-content position-absolute" style="display: none; outline: none;"> <div class="Popover-message Popover-message--bottom-left Popover-message--large Box color-shadow-large" style="width:360px;"> </div> </div> <template id="snippet-clipboard-copy-button"> <div class="zeroclipboard-container position-absolute right-0 top-0"> <clipboard-copy aria-label="Copy" class="ClipboardButton btn js-clipboard-copy m-2 p-0" data-copy-feedback="Copied!" data-tooltip-direction="w"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-copy js-clipboard-copy-icon m-2"> <path d="M0 6.75C0 5.784.784 5 1.75 5h1.5a.75.75 0 0 1 0 1.5h-1.5a.25.25 0 0 0-.25.25v7.5c0 .138.112.25.25.25h7.5a.25.25 0 0 0 .25-.25v-1.5a.75.75 0 0 1 1.5 0v1.5A1.75 1.75 0 0 1 9.25 16h-7.5A1.75 1.75 0 0 1 0 14.25Z"></path><path d="M5 1.75C5 .784 5.784 0 6.75 0h7.5C15.216 0 16 .784 16 1.75v7.5A1.75 1.75 0 0 1 14.25 11h-7.5A1.75 1.75 0 0 1 5 9.25Zm1.75-.25a.25.25 0 0 0-.25.25v7.5c0 .138.112.25.25.25h7.5a.25.25 0 0 0 .25-.25v-7.5a.25.25 0 0 0-.25-.25Z"></path> </svg> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-check js-clipboard-check-icon color-fg-success d-none m-2"> <path d="M13.78 4.22a.75.75 0 0 1 0 1.06l-7.25 7.25a.75.75 0 0 1-1.06 0L2.22 9.28a.751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018L6 10.94l6.72-6.72a.75.75 0 0 1 1.06 0Z"></path> </svg> </clipboard-copy> </div> </template> <template id="snippet-clipboard-copy-button-unpositioned"> <div class="zeroclipboard-container"> <clipboard-copy aria-label="Copy" class="ClipboardButton btn btn-invisible js-clipboard-copy m-2 p-0 d-flex flex-justify-center flex-items-center" data-copy-feedback="Copied!" data-tooltip-direction="w"> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-copy js-clipboard-copy-icon"> <path d="M0 6.75C0 5.784.784 5 1.75 5h1.5a.75.75 0 0 1 0 1.5h-1.5a.25.25 0 0 0-.25.25v7.5c0 .138.112.25.25.25h7.5a.25.25 0 0 0 .25-.25v-1.5a.75.75 0 0 1 1.5 0v1.5A1.75 1.75 0 0 1 9.25 16h-7.5A1.75 1.75 0 0 1 0 14.25Z"></path><path d="M5 1.75C5 .784 5.784 0 6.75 0h7.5C15.216 0 16 .784 16 1.75v7.5A1.75 1.75 0 0 1 14.25 11h-7.5A1.75 1.75 0 0 1 5 9.25Zm1.75-.25a.25.25 0 0 0-.25.25v7.5c0 .138.112.25.25.25h7.5a.25.25 0 0 0 .25-.25v-7.5a.25.25 0 0 0-.25-.25Z"></path> </svg> <svg aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-check js-clipboard-check-icon color-fg-success d-none"> <path d="M13.78 4.22a.75.75 0 0 1 0 1.06l-7.25 7.25a.75.75 0 0 1-1.06 0L2.22 9.28a.751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018L6 10.94l6.72-6.72a.75.75 0 0 1 1.06 0Z"></path> </svg> </clipboard-copy> </div> </template> </div> <div id="js-global-screen-reader-notice" class="sr-only mt-n1" aria-live="polite" aria-atomic="true" ></div> <div id="js-global-screen-reader-notice-assertive" class="sr-only mt-n1" aria-live="assertive" aria-atomic="true"></div> </body> </html>