🕷️ Crawler Inspector

URL Lookup

Direct Parameter Lookup

Raw Queries and Responses

1. Shard Calculation

Query:

Response:

Calculated Shard: 102 (from laksa182)

2. Crawled Status Check

Query:

curl -X POST \
  'http://laksa102.int.ahrefs:8124/' \
  -H 'Content-Type: text/plain' \
  -H 'X-ClickHouse-Database: crawler3' \
  -H 'Authorization: Basic YXBpOg==' \
  -d 'SELECT getAhrefsURLFromUnparsed(src_unparsed) AS found_url, ifNull(toUnixTimestamp(download_stamp), 0) AS crawl_time, ifNull(toUnixTimestamp(props_url_first_seen), 0) AS first_indexed_time, download_http_code AS http_code, src_unparsed AS src_unparsed, src_root_hash AS src_root_hash, history_drop_reason AS history_drop_reason, meta_title AS meta_title, meta_descriptions AS meta_descriptions, attrs_boilerpipe_text AS attrs_boilerpipe_text, attrs_markdown AS attrs_markdown, attrs_readable_markdown AS attrs_readable_markdown, meta_canonical AS meta_canonical FROM crawler3.page_info_local FINAL PREWHERE (src_root_hash, src_unparsed) IN ((getAhrefsRootHashFromUnparsed(getAhrefsUnparsedNoserviceFromURL(\'https://en.wikibooks.org/wiki/Python_Programming/Metaclasses\')), getAhrefsUnparsedNoserviceFromURL(\'https://en.wikibooks.org/wiki/Python_Programming/Metaclasses\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses","crawl_time":1775242356,"first_indexed_time":1435544014,"http_code":200,"src_unparsed":"org,wikibooks!en,\/wiki\/Python_Programming\/Metaclasses s443","src_root_hash":"7794694764302024702","history_drop_reason":null,"meta_title":"Python Programming\/Metaclasses - Wikibooks, open books for an open world","meta_descriptions":[],"attrs_boilerpipe_text":"From Wikibooks, open books for an open world\nIn Python, classes are themselves objects.  Just as other objects are instances of a particular class, classes themselves are instances of a metaclass.\nThe\nPep 3115\ndefines the changes to python 3 metaclasses.\nIn python3 you have a method __prepare__ that is called in the metaclass to create a dictionary or other class to store the class members.\n[\n1\n]\nThen there is the __new__ method that is called to create new instances of that class.\n[\n2\n]\nThe metaclass for all standard Python types is the \"type\" object.\n>>>\ntype\n(\nobject\n)\n<\ntype\n'type'\n>\n>>>\ntype\n(\nint\n)\n<\ntype\n'type'\n>\n>>>\ntype\n(\nlist\n)\n<\ntype\n'type'\n>\nJust like list, int and object, \"type\" is itself a normal Python object, and is itself an instance of a class.  In this case, it is in fact an instance of itself.\n>>>\ntype\n(\ntype\n)\n<\ntype\n'type'\n>\nIt can be instantiated to create new class objects similarly to the class factory example above by passing the name of the new class, the base classes to inherit from, and a dictionary defining the namespace to use.\nFor instance, the code:\n>>>\nclass\nMyClass\n(\nBaseClass\n):\n...\nattribute\n=\n42\nCould also be written as:\n>>>\nMyClass\n=\ntype\n(\n\"MyClass\"\n,\n(\nBaseClass\n,),\n{\n'attribute'\n:\n42\n})\nIt is possible to create a class with a different metaclass than type by setting the metaclass keyword argument when defining the class.  When this is done, the class, and its subclass will be created using your custom metaclass.  For example\nclass\nCustomMetaclass\n(\ntype\n):\ndef\n__init__\n(\ncls\n,\nname\n,\nbases\n,\ndct\n):\nprint\n(\n\"Creating class\n%s\nusing CustomMetaclass\"\n%\nname\n)\nsuper\n(\nCustomMetaclass\n,\ncls\n)\n.\n__init__\n(\nname\n,\nbases\n,\ndct\n)\nclass\nBaseClass\n(\nmetaclass\n=\nCustomMetaclass\n):\npass\nclass\nSubclass1\n(\nBaseClass\n):\npass\nThis will print\nCreating class BaseClass using CustomMetaclass\nCreating class Subclass1 using CustomMetaclass\nBy creating a custom metaclass in this way, it is possible to change how the class is constructed.  This allows you to add or remove attributes and methods, register creation of classes and subclasses creation and various other manipulations when the class is created.\nWikipedia article on Aspect Oriented Programming\nUnifying types and classes in Python 2.2\nO'Reilly Article on Python Metaclasses\n↑\nhttp:\/\/www.python.org\/dev\/peps\/pep-3115\/\n↑\nhttp:\/\/eli.thegreenplace.net\/2011\/08\/14\/python-metaclasses-by-example\/\nTo do:\n[Incomplete] (see Putting Metaclasses to Work, Ira R. Forman, Scott H. Danforth?)","attrs_markdown":"[Jump to content](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#bodyContent)\n\nMain menu\n\nMain menu\n\nmove to sidebar\n\nhide\n\nNavigation\n\n- [Main Page](https:\/\/en.wikibooks.org\/wiki\/Main_Page \"Visit the main page [alt-z]\")\n- [Help](https:\/\/en.wikibooks.org\/wiki\/Help:Contents \"Find help on how to use and edit Wikibooks\")\n- [Browse](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:Card_Catalog_Office \"Check out what Wikibooks has to offer\")\n- [Cookbook](https:\/\/en.wikibooks.org\/wiki\/Cookbook:Table_of_Contents \"Learn recipes from around the world\")\n- [Wikijunior](https:\/\/en.wikibooks.org\/wiki\/Wikijunior \"Books for children\")\n- [Featured books](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:Featured_books \"The best of Wikibooks\")\n- [Recent changes](https:\/\/en.wikibooks.org\/wiki\/Special:RecentChanges \"A list of recent changes in the wiki [alt-r]\")\n- [Special pages](https:\/\/en.wikibooks.org\/wiki\/Special:SpecialPages \"A list of all special pages [alt-q]\")\n- [Random book](https:\/\/en.wikibooks.org\/wiki\/Special:RandomInCategory\/Book:Wikibooks_Stacks\/Books)\n- [Using Wikibooks](https:\/\/en.wikibooks.org\/wiki\/Using_Wikibooks)\n\nCommunity\n\n- [Reading room forum](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:Reading_room)\n- [Community portal](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:Community_Portal \"Find your way around the Wikibooks community\")\n- [Help out\\!](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:Maintenance \"Frequent tasks that you can help with\")\n- [Policies and guidelines](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:Policies_and_guidelines \"Pages detailing important rules and procedures\")\n- [Contact us](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:Contact_us \"Alternative methods of communication\")\n\n[![](https:\/\/en.wikibooks.org\/static\/images\/icons\/wikibooks.svg) ![Wikibooks](https:\/\/en.wikibooks.org\/static\/images\/mobile\/copyright\/wikibooks-wordmark-vi.svg) ![The Free Textbook Project](https:\/\/en.wikibooks.org\/static\/images\/mobile\/copyright\/wikibooks-tagline-en.svg)](https:\/\/en.wikibooks.org\/wiki\/Main_Page)\n\n[Search](https:\/\/en.wikibooks.org\/wiki\/Special:Search \"Search Wikibooks [alt-f]\")\n\nAppearance\n\nAppearance\n\nmove to sidebar\n\nhide\n\nText\n\nThis page always uses small font size\n\nWidth\n\nThe content is as wide as possible for your browser window.\n\nColor (beta)\n\nThis page is always in light mode.\n\n- [Donations](https:\/\/donate.wikimedia.org\/?wmf_source=donate&wmf_medium=sidebar&wmf_campaign=en.wikibooks.org&uselang=en)\n- [Create account](https:\/\/en.wikibooks.org\/w\/index.php?title=Special:CreateAccount&returnto=Python+Programming%2FMetaclasses \"You are encouraged to create an account and log in; however, it is not mandatory\")\n- [Log in](https:\/\/en.wikibooks.org\/w\/index.php?title=Special:UserLogin&returnto=Python+Programming%2FMetaclasses \"You are encouraged to log in; however, it is not mandatory [alt-o]\")\n\nPersonal tools\n\n- [Donations](https:\/\/donate.wikimedia.org\/?wmf_source=donate&wmf_medium=sidebar&wmf_campaign=en.wikibooks.org&uselang=en)\n- [Create account](https:\/\/en.wikibooks.org\/w\/index.php?title=Special:CreateAccount&returnto=Python+Programming%2FMetaclasses \"You are encouraged to create an account and log in; however, it is not mandatory\")\n- [Log in](https:\/\/en.wikibooks.org\/w\/index.php?title=Special:UserLogin&returnto=Python+Programming%2FMetaclasses \"You are encouraged to log in; however, it is not mandatory [alt-o]\")\n\nToggle the table of contents\n\n## Contents\nmove to sidebar\n\nhide\n\n- [Beginning](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses)\n- [1 Python3](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#Python3)\n- [2 The type Metaclass](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#The_type_Metaclass)\n- [3 Metaclasses](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#Metaclasses)\n- [4 More resources](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#More_resources)\n- [5 References](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#References)\n# Python Programming\/Metaclasses\n1 language\n\n- [中文](https:\/\/zh.wikibooks.org\/wiki\/Python\/%E5%85%83%E7%B1%BB \"Python\/元类 – Chinese\")\n\n[Edit links](https:\/\/www.wikidata.org\/wiki\/Special:EntityPage\/Q96920717#sitelinks-wikibooks \"Edit interlanguage links\")\n\n- [Book](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses \"View the content page [alt-c]\")\n- [Discussion](https:\/\/en.wikibooks.org\/wiki\/Talk:Python_Programming\/Metaclasses \"Discussion about the content page [alt-t]\")\n\nEnglish\n\n- [Read](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses)\n- [Edit](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&veaction=edit \"Edit this page [alt-v]\")\n- [Edit source](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=edit \"Edit the source code of this page [alt-e]\")\n- [View history](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=history \"Past revisions of this page [alt-h]\")\n\nTools\n\nTools\n\nmove to sidebar\n\nhide\n\nActions\n\n- [Read](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses)\n- [Edit](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&veaction=edit \"Edit this page [alt-v]\")\n- [Edit source](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=edit \"Edit the source code of this page [alt-e]\")\n- [View history](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=history)\n\nGeneral\n\n- [What links here](https:\/\/en.wikibooks.org\/wiki\/Special:WhatLinksHere\/Python_Programming\/Metaclasses \"A list of all wiki pages that link here [alt-j]\")\n- [Related changes](https:\/\/en.wikibooks.org\/wiki\/Special:RecentChangesLinked\/Python_Programming\/Metaclasses \"Recent changes in pages linked from this page [alt-k]\")\n- [Upload file](https:\/\/commons.wikimedia.org\/wiki\/Special:UploadWizard?uselang=en \"Upload files [alt-u]\")\n- [Permanent link](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&oldid=4070786 \"Permanent link to this revision of this page\")\n- [Page information](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=info \"More information about this page\")\n- [Cite this page](https:\/\/en.wikibooks.org\/w\/index.php?title=Special:CiteThisPage&page=Python_Programming%2FMetaclasses&id=4070786&wpFormIdentifier=titleform \"Information on how to cite this page\")\n- [Get shortened URL](https:\/\/en.wikibooks.org\/w\/index.php?title=Special:UrlShortener&url=https%3A%2F%2Fen.wikibooks.org%2Fwiki%2FPython_Programming%2FMetaclasses)\n- [Edit interlanguage links](https:\/\/www.wikidata.org\/wiki\/Special:EntityPage\/Q96920717#sitelinks-wikibooks \"Edit interlanguage links\")\n\nSister projects\n\n- [Wikipedia](https:\/\/en.wikipedia.org\/wiki\/Main_Page)\n- [Wikiversity](https:\/\/en.wikiversity.org\/wiki\/Wikiversity:Main_Page)\n- [Wiktionary](https:\/\/en.wiktionary.org\/wiki\/Wiktionary:Main_Page)\n- [Wikiquote](https:\/\/en.wikiquote.org\/wiki\/Main_Page)\n- [Wikisource](https:\/\/en.wikisource.org\/wiki\/Main_Page)\n- [Wikinews](https:\/\/en.wikinews.org\/wiki\/Main_Page)\n- [Wikivoyage](https:\/\/en.wikivoyage.org\/wiki\/Main_Page)\n- [Commons](https:\/\/commons.wikimedia.org\/wiki\/Main_Page)\n- [Wikidata](https:\/\/www.wikidata.org\/wiki\/Wikidata:Main_Page)\n- [MediaWiki](https:\/\/www.mediawiki.org\/wiki\/Main_Page)\n- [Meta-Wiki](https:\/\/meta.wikimedia.org\/wiki\/Main_Page)\n\nPrint\/export\n\n- [Create a collection](https:\/\/en.wikibooks.org\/w\/index.php?title=Special:Book&bookcmd=book_creator&referer=Python+Programming%2FMetaclasses)\n- [Download as PDF](https:\/\/en.wikibooks.org\/w\/index.php?title=Special:DownloadAsPdf&page=Python_Programming%2FMetaclasses&action=show-download-screen)\n- [Printable version](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&printable=yes \"Printable version of this page [alt-p]\")\n\nIn other projects\n\n- [Wikidata item](https:\/\/www.wikidata.org\/wiki\/Special:EntityPage\/Q96920717 \"Link to connected data repository item [alt-g]\")\n\nFrom Wikibooks, open books for an open world\n\n\\<\n\n[Python Programming](https:\/\/en.wikibooks.org\/wiki\/Python_Programming \"Python Programming\")\n\n|   |   |   |\n|---|---|---|\n| [Previous: Reflection](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Reflection \"Python Programming\/Reflection\") | [Index](https:\/\/en.wikibooks.org\/wiki\/Python_Programming \"Python Programming\") | [Next: Tips and Tricks](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Tips_and_Tricks \"Python Programming\/Tips and Tricks\") |\n\nIn Python, classes are themselves objects. Just as other objects are instances of a particular class, classes themselves are instances of a metaclass.\n\n### Python3\n\\[[edit](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&veaction=edit&section=1 \"Edit section: Python3\") \\| [edit source](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=edit&section=1 \"Edit section's source code: Python3\")\\]\n\nThe [Pep 3115](https:\/\/www.python.org\/dev\/peps\/pep-3115\/) defines the changes to python 3 metaclasses. In python3 you have a method \\_\\_prepare\\_\\_ that is called in the metaclass to create a dictionary or other class to store the class members.[\\[1\\]](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#cite_note-1) Then there is the \\_\\_new\\_\\_ method that is called to create new instances of that class. [\\[2\\]](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#cite_note-2)\n\n### The type Metaclass\n\\[[edit](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&veaction=edit&section=2 \"Edit section: The type Metaclass\") \\| [edit source](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=edit&section=2 \"Edit section's source code: The type Metaclass\")\\]\n\nThe metaclass for all standard Python types is the \"type\" object.\n```\n>>> type(object)\n<type 'type'>\n>>> type(int)\n<type 'type'>\n>>> type(list)\n<type 'type'>\n```\nJust like list, int and object, \"type\" is itself a normal Python object, and is itself an instance of a class. In this case, it is in fact an instance of itself.\n```\n>>> type(type)\n<type 'type'>\n```\nIt can be instantiated to create new class objects similarly to the class factory example above by passing the name of the new class, the base classes to inherit from, and a dictionary defining the namespace to use.\n\nFor instance, the code:\n```\n>>> class MyClass(BaseClass):\n...     attribute = 42\n```\nCould also be written as:\n```\n>>> MyClass = type(\"MyClass\", (BaseClass,), {'attribute' : 42})\n```\n### Metaclasses\n\\[[edit](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&veaction=edit&section=3 \"Edit section: Metaclasses\") \\| [edit source](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=edit&section=3 \"Edit section's source code: Metaclasses\")\\]\n\nIt is possible to create a class with a different metaclass than type by setting the metaclass keyword argument when defining the class. When this is done, the class, and its subclass will be created using your custom metaclass. For example\n```\nclass CustomMetaclass(type):\n    def __init__(cls, name, bases, dct):\n        print(\"Creating class %s using CustomMetaclass\" % name)\n        super(CustomMetaclass, cls).__init__(name, bases, dct)\n\nclass BaseClass(metaclass=CustomMetaclass):\n    pass\n\nclass Subclass1(BaseClass):\n    pass\n```\nThis will print\n```\nCreating class BaseClass using CustomMetaclass\nCreating class Subclass1 using CustomMetaclass\n```\nBy creating a custom metaclass in this way, it is possible to change how the class is constructed. This allows you to add or remove attributes and methods, register creation of classes and subclasses creation and various other manipulations when the class is created.\n\n### More resources\n\\[[edit](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&veaction=edit&section=4 \"Edit section: More resources\") \\| [edit source](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=edit&section=4 \"Edit section's source code: More resources\")\\]\n\n- [Wikipedia article on Aspect Oriented Programming](https:\/\/en.wikipedia.org\/wiki\/Aspect-oriented_programming \"w:Aspect-oriented programming\")\n- [Unifying types and classes in Python 2.2](https:\/\/www.python.org\/2.2\/descrintro.html)\n- [O'Reilly Article on Python Metaclasses](http:\/\/www.onlamp.com\/pub\/a\/python\/2003\/04\/17\/metaclasses.html)\n\n### References\n\\[[edit](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&veaction=edit&section=5 \"Edit section: References\") \\| [edit source](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&action=edit&section=5 \"Edit section's source code: References\")\\]\n\n1. [↑](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#cite_ref-1 \"Jump up\") [http:\/\/www.python.org\/dev\/peps\/pep-3115\/](https:\/\/www.python.org\/dev\/peps\/pep-3115\/)\n2. [↑](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#cite_ref-2 \"Jump up\") <http:\/\/eli.thegreenplace.net\/2011\/08\/14\/python-metaclasses-by-example\/>\n\n|   |   |\n|---|---|\n| ![Clipboard](https:\/\/upload.wikimedia.org\/wikipedia\/commons\/thumb\/1\/1f\/Clipboard.svg\/60px-Clipboard.svg.png) | **To do:** \\[Incomplete\\] (see Putting Metaclasses to Work, Ira R. Forman, Scott H. Danforth?) |\n\n|   |   |   |\n|---|---|---|\n| [Previous: Reflection](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Reflection \"Python Programming\/Reflection\") | [Index](https:\/\/en.wikibooks.org\/wiki\/Python_Programming \"Python Programming\") | [Next: Tips and Tricks](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Tips_and_Tricks \"Python Programming\/Tips and Tricks\") |\n\n![](https:\/\/en.wikibooks.org\/wiki\/Special:CentralAutoLogin\/start?useformat=desktop&type=1x1&usesul3=1)\n\nRetrieved from \"<https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&oldid=4070786>\"\n\n[Category](https:\/\/en.wikibooks.org\/wiki\/Special:Categories \"Special:Categories\"):\n\n- [Book:Python Programming](https:\/\/en.wikibooks.org\/wiki\/Category:Book:Python_Programming \"Category:Book:Python Programming\")\n\nHidden category:\n\n- [Wikibooks pages with to-do lists](https:\/\/en.wikibooks.org\/wiki\/Category:Wikibooks_pages_with_to-do_lists \"Category:Wikibooks pages with to-do lists\")\n\n- This page was last edited on 5 June 2022, at 08:10.\n- Text is available under the [Creative Commons Attribution-ShareAlike License](https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/); additional terms may apply. By using this site, you agree to the [Terms of Use](https:\/\/foundation.wikimedia.org\/wiki\/Special:MyLanguage\/Policy:Terms_of_Use) and [Privacy Policy.](https:\/\/foundation.wikimedia.org\/wiki\/Special:MyLanguage\/Policy:Privacy_policy)\n\n- [Privacy policy](https:\/\/foundation.wikimedia.org\/wiki\/Special:MyLanguage\/Policy:Privacy_policy)\n- [About Wikibooks](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:Welcome)\n- [Disclaimers](https:\/\/en.wikibooks.org\/wiki\/Wikibooks:General_disclaimer)\n- [Code of Conduct](https:\/\/foundation.wikimedia.org\/wiki\/Special:MyLanguage\/Policy:Universal_Code_of_Conduct)\n- [Developers](https:\/\/developer.wikimedia.org\/)\n- [Statistics](https:\/\/stats.wikimedia.org\/#\/en.wikibooks.org)\n- [Cookie statement](https:\/\/foundation.wikimedia.org\/wiki\/Special:MyLanguage\/Policy:Cookie_statement)\n- [Mobile view](https:\/\/en.wikibooks.org\/w\/index.php?title=Python_Programming\/Metaclasses&mobileaction=toggle_view_mobile)\n\n- [![Wikimedia Foundation](https:\/\/en.wikibooks.org\/static\/images\/footer\/wikimedia.svg)](https:\/\/www.wikimedia.org\/)\n- [![Powered by MediaWiki](https:\/\/en.wikibooks.org\/w\/resources\/assets\/mediawiki_compact.svg)](https:\/\/www.mediawiki.org\/)\n\nSearch\n\nToggle the table of contents\n\nPython Programming\/Metaclasses\n\n1 language\n\n[Add topic](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses)","attrs_readable_markdown":"From Wikibooks, open books for an open world\n\nIn Python, classes are themselves objects. Just as other objects are instances of a particular class, classes themselves are instances of a metaclass.\n\nThe [Pep 3115](https:\/\/www.python.org\/dev\/peps\/pep-3115\/) defines the changes to python 3 metaclasses. In python3 you have a method \\_\\_prepare\\_\\_ that is called in the metaclass to create a dictionary or other class to store the class members.[\\[1\\]](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#cite_note-1) Then there is the \\_\\_new\\_\\_ method that is called to create new instances of that class. [\\[2\\]](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#cite_note-2)\n\nThe metaclass for all standard Python types is the \"type\" object.\n```\n>>> type(object)\n<type 'type'>\n>>> type(int)\n<type 'type'>\n>>> type(list)\n<type 'type'>\n```\nJust like list, int and object, \"type\" is itself a normal Python object, and is itself an instance of a class. In this case, it is in fact an instance of itself.\n```\n>>> type(type)\n<type 'type'>\n```\nIt can be instantiated to create new class objects similarly to the class factory example above by passing the name of the new class, the base classes to inherit from, and a dictionary defining the namespace to use.\n\nFor instance, the code:\n```\n>>> class MyClass(BaseClass):\n...     attribute = 42\n```\nCould also be written as:\n```\n>>> MyClass = type(\"MyClass\", (BaseClass,), {'attribute' : 42})\n```\nIt is possible to create a class with a different metaclass than type by setting the metaclass keyword argument when defining the class. When this is done, the class, and its subclass will be created using your custom metaclass. For example\n```\nclass CustomMetaclass(type):\n    def __init__(cls, name, bases, dct):\n        print(\"Creating class %s using CustomMetaclass\" % name)\n        super(CustomMetaclass, cls).__init__(name, bases, dct)\n\nclass BaseClass(metaclass=CustomMetaclass):\n    pass\n\nclass Subclass1(BaseClass):\n    pass\n```\nThis will print\n```\nCreating class BaseClass using CustomMetaclass\nCreating class Subclass1 using CustomMetaclass\n```\nBy creating a custom metaclass in this way, it is possible to change how the class is constructed. This allows you to add or remove attributes and methods, register creation of classes and subclasses creation and various other manipulations when the class is created.\n\n- [Wikipedia article on Aspect Oriented Programming](https:\/\/en.wikipedia.org\/wiki\/Aspect-oriented_programming \"w:Aspect-oriented programming\")\n- [Unifying types and classes in Python 2.2](https:\/\/www.python.org\/2.2\/descrintro.html)\n- [O'Reilly Article on Python Metaclasses](http:\/\/www.onlamp.com\/pub\/a\/python\/2003\/04\/17\/metaclasses.html)\n\n1. [↑](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#cite_ref-1 \"Jump up\") [http:\/\/www.python.org\/dev\/peps\/pep-3115\/](https:\/\/www.python.org\/dev\/peps\/pep-3115\/)\n2. [↑](https:\/\/en.wikibooks.org\/wiki\/Python_Programming\/Metaclasses#cite_ref-2 \"Jump up\") <http:\/\/eli.thegreenplace.net\/2011\/08\/14\/python-metaclasses-by-example\/>\n\n|   |   |\n|---|---|\n| ![Clipboard](https:\/\/upload.wikimedia.org\/wikipedia\/commons\/thumb\/1\/1f\/Clipboard.svg\/60px-Clipboard.svg.png) | **To do:** \\[Incomplete\\] (see Putting Metaclasses to Work, Ira R. Forman, Scott H. Danforth?) |","meta_canonical":null}

3. Robots.txt Check

Query:

Response:

4. Spam/Ban Check

Query:

Response:

5. Seen Status Check

ℹ️ Skipped - page is already crawled

📄

INDEXABLE

✅

CRAWLED

9 days ago

🤖

ROBOTS ALLOWED

Page Info Filters

Filter	Status	Condition	Details
HTTP status	PASS	`download_http_code = 200`	HTTP 200
Age cutoff	PASS	`download_stamp > now() - 6 MONTH`	0.3 months ago
History drop	PASS	`isNull(history_drop_reason)`	No drop reason
Spam/ban	PASS	`fh_dont_index != 1 AND ml_spam_score = 0`	ml_spam_score=0
Canonical	PASS	`meta_canonical IS NULL OR = '' OR = src_unparsed`	Not set

Page Details

Property	Value
URL	https://en.wikibooks.org/wiki/Python_Programming/Metaclasses
Last Crawled	2026-04-03 18:52:36 (9 days ago)
First Indexed	2015-06-29 02:13:34 (10 years ago)
HTTP Status Code	200
Meta Title	Python Programming/Metaclasses - Wikibooks, open books for an open world
Meta Description	null
Meta Canonical	null
Boilerpipe Text	From Wikibooks, open books for an open world In Python, classes are themselves objects. Just as other objects are instances of a particular class, classes themselves are instances of a metaclass. The Pep 3115 defines the changes to python 3 metaclasses. In python3 you have a method __prepare__ that is called in the metaclass to create a dictionary or other class to store the class members. [ 1 ] Then there is the __new__ method that is called to create new instances of that class. [ 2 ] The metaclass for all standard Python types is the "type" object. >>> type ( object ) < type 'type' > >>> type ( int ) < type 'type' > >>> type ( list ) < type 'type' > Just like list, int and object, "type" is itself a normal Python object, and is itself an instance of a class. In this case, it is in fact an instance of itself. >>> type ( type ) < type 'type' > It can be instantiated to create new class objects similarly to the class factory example above by passing the name of the new class, the base classes to inherit from, and a dictionary defining the namespace to use. For instance, the code: >>> class MyClass ( BaseClass ): ... attribute = 42 Could also be written as: >>> MyClass = type ( "MyClass" , ( BaseClass ,), { 'attribute' : 42 }) It is possible to create a class with a different metaclass than type by setting the metaclass keyword argument when defining the class. When this is done, the class, and its subclass will be created using your custom metaclass. For example class CustomMetaclass ( type ): def __init__ ( cls , name , bases , dct ): print ( "Creating class %s using CustomMetaclass" % name ) super ( CustomMetaclass , cls ) . __init__ ( name , bases , dct ) class BaseClass ( metaclass = CustomMetaclass ): pass class Subclass1 ( BaseClass ): pass This will print Creating class BaseClass using CustomMetaclass Creating class Subclass1 using CustomMetaclass By creating a custom metaclass in this way, it is possible to change how the class is constructed. This allows you to add or remove attributes and methods, register creation of classes and subclasses creation and various other manipulations when the class is created. Wikipedia article on Aspect Oriented Programming Unifying types and classes in Python 2.2 O'Reilly Article on Python Metaclasses ↑ http://www.python.org/dev/peps/pep-3115/ ↑ http://eli.thegreenplace.net/2011/08/14/python-metaclasses-by-example/ To do: [Incomplete] (see Putting Metaclasses to Work, Ira R. Forman, Scott H. Danforth?)
Markdown	[Jump to content](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#bodyContent) Main menu Main menu move to sidebar hide Navigation - [Main Page](https://en.wikibooks.org/wiki/Main_Page "Visit the main page [alt-z]") - [Help](https://en.wikibooks.org/wiki/Help:Contents "Find help on how to use and edit Wikibooks") - [Browse](https://en.wikibooks.org/wiki/Wikibooks:Card_Catalog_Office "Check out what Wikibooks has to offer") - [Cookbook](https://en.wikibooks.org/wiki/Cookbook:Table_of_Contents "Learn recipes from around the world") - [Wikijunior](https://en.wikibooks.org/wiki/Wikijunior "Books for children") - [Featured books](https://en.wikibooks.org/wiki/Wikibooks:Featured_books "The best of Wikibooks") - [Recent changes](https://en.wikibooks.org/wiki/Special:RecentChanges "A list of recent changes in the wiki [alt-r]") - [Special pages](https://en.wikibooks.org/wiki/Special:SpecialPages "A list of all special pages [alt-q]") - [Random book](https://en.wikibooks.org/wiki/Special:RandomInCategory/Book:Wikibooks_Stacks/Books) - [Using Wikibooks](https://en.wikibooks.org/wiki/Using_Wikibooks) Community - [Reading room forum](https://en.wikibooks.org/wiki/Wikibooks:Reading_room) - [Community portal](https://en.wikibooks.org/wiki/Wikibooks:Community_Portal "Find your way around the Wikibooks community") - [Help out\!](https://en.wikibooks.org/wiki/Wikibooks:Maintenance "Frequent tasks that you can help with") - [Policies and guidelines](https://en.wikibooks.org/wiki/Wikibooks:Policies_and_guidelines "Pages detailing important rules and procedures") - [Contact us](https://en.wikibooks.org/wiki/Wikibooks:Contact_us "Alternative methods of communication") [![](https://en.wikibooks.org/static/images/icons/wikibooks.svg) ![Wikibooks](https://en.wikibooks.org/static/images/mobile/copyright/wikibooks-wordmark-vi.svg) ![The Free Textbook Project](https://en.wikibooks.org/static/images/mobile/copyright/wikibooks-tagline-en.svg)](https://en.wikibooks.org/wiki/Main_Page) [Search](https://en.wikibooks.org/wiki/Special:Search "Search Wikibooks [alt-f]") Appearance Appearance move to sidebar hide Text This page always uses small font size Width The content is as wide as possible for your browser window. Color (beta) This page is always in light mode. - [Donations](https://donate.wikimedia.org/?wmf_source=donate&wmf_medium=sidebar&wmf_campaign=en.wikibooks.org&uselang=en) - [Create account](https://en.wikibooks.org/w/index.php?title=Special:CreateAccount&returnto=Python+Programming%2FMetaclasses "You are encouraged to create an account and log in; however, it is not mandatory") - [Log in](https://en.wikibooks.org/w/index.php?title=Special:UserLogin&returnto=Python+Programming%2FMetaclasses "You are encouraged to log in; however, it is not mandatory [alt-o]") Personal tools - [Donations](https://donate.wikimedia.org/?wmf_source=donate&wmf_medium=sidebar&wmf_campaign=en.wikibooks.org&uselang=en) - [Create account](https://en.wikibooks.org/w/index.php?title=Special:CreateAccount&returnto=Python+Programming%2FMetaclasses "You are encouraged to create an account and log in; however, it is not mandatory") - [Log in](https://en.wikibooks.org/w/index.php?title=Special:UserLogin&returnto=Python+Programming%2FMetaclasses "You are encouraged to log in; however, it is not mandatory [alt-o]") Toggle the table of contents ## Contents move to sidebar hide - [Beginning](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses) - [1 Python3](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#Python3) - [2 The type Metaclass](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#The_type_Metaclass) - [3 Metaclasses](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#Metaclasses) - [4 More resources](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#More_resources) - [5 References](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#References) # Python Programming/Metaclasses 1 language - [中文](https://zh.wikibooks.org/wiki/Python/%E5%85%83%E7%B1%BB "Python/元类 – Chinese") [Edit links](https://www.wikidata.org/wiki/Special:EntityPage/Q96920717#sitelinks-wikibooks "Edit interlanguage links") - [Book](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses "View the content page [alt-c]") - [Discussion](https://en.wikibooks.org/wiki/Talk:Python_Programming/Metaclasses "Discussion about the content page [alt-t]") English - [Read](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses) - [Edit](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&veaction=edit "Edit this page [alt-v]") - [Edit source](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=edit "Edit the source code of this page [alt-e]") - [View history](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=history "Past revisions of this page [alt-h]") Tools Tools move to sidebar hide Actions - [Read](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses) - [Edit](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&veaction=edit "Edit this page [alt-v]") - [Edit source](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=edit "Edit the source code of this page [alt-e]") - [View history](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=history) General - [What links here](https://en.wikibooks.org/wiki/Special:WhatLinksHere/Python_Programming/Metaclasses "A list of all wiki pages that link here [alt-j]") - [Related changes](https://en.wikibooks.org/wiki/Special:RecentChangesLinked/Python_Programming/Metaclasses "Recent changes in pages linked from this page [alt-k]") - [Upload file](https://commons.wikimedia.org/wiki/Special:UploadWizard?uselang=en "Upload files [alt-u]") - [Permanent link](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&oldid=4070786 "Permanent link to this revision of this page") - [Page information](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=info "More information about this page") - [Cite this page](https://en.wikibooks.org/w/index.php?title=Special:CiteThisPage&page=Python_Programming%2FMetaclasses&id=4070786&wpFormIdentifier=titleform "Information on how to cite this page") - [Get shortened URL](https://en.wikibooks.org/w/index.php?title=Special:UrlShortener&url=https%3A%2F%2Fen.wikibooks.org%2Fwiki%2FPython_Programming%2FMetaclasses) - [Edit interlanguage links](https://www.wikidata.org/wiki/Special:EntityPage/Q96920717#sitelinks-wikibooks "Edit interlanguage links") Sister projects - [Wikipedia](https://en.wikipedia.org/wiki/Main_Page) - [Wikiversity](https://en.wikiversity.org/wiki/Wikiversity:Main_Page) - [Wiktionary](https://en.wiktionary.org/wiki/Wiktionary:Main_Page) - [Wikiquote](https://en.wikiquote.org/wiki/Main_Page) - [Wikisource](https://en.wikisource.org/wiki/Main_Page) - [Wikinews](https://en.wikinews.org/wiki/Main_Page) - [Wikivoyage](https://en.wikivoyage.org/wiki/Main_Page) - [Commons](https://commons.wikimedia.org/wiki/Main_Page) - [Wikidata](https://www.wikidata.org/wiki/Wikidata:Main_Page) - [MediaWiki](https://www.mediawiki.org/wiki/Main_Page) - [Meta-Wiki](https://meta.wikimedia.org/wiki/Main_Page) Print/export - [Create a collection](https://en.wikibooks.org/w/index.php?title=Special:Book&bookcmd=book_creator&referer=Python+Programming%2FMetaclasses) - [Download as PDF](https://en.wikibooks.org/w/index.php?title=Special:DownloadAsPdf&page=Python_Programming%2FMetaclasses&action=show-download-screen) - [Printable version](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&printable=yes "Printable version of this page [alt-p]") In other projects - [Wikidata item](https://www.wikidata.org/wiki/Special:EntityPage/Q96920717 "Link to connected data repository item [alt-g]") From Wikibooks, open books for an open world \< [Python Programming](https://en.wikibooks.org/wiki/Python_Programming "Python Programming") \| \| \| \| \|---\|---\|---\| \| [Previous: Reflection](https://en.wikibooks.org/wiki/Python_Programming/Reflection "Python Programming/Reflection") \| [Index](https://en.wikibooks.org/wiki/Python_Programming "Python Programming") \| [Next: Tips and Tricks](https://en.wikibooks.org/wiki/Python_Programming/Tips_and_Tricks "Python Programming/Tips and Tricks") \| In Python, classes are themselves objects. Just as other objects are instances of a particular class, classes themselves are instances of a metaclass. ### Python3 \[[edit](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&veaction=edit&section=1 "Edit section: Python3") \\| [edit source](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=edit&section=1 "Edit section's source code: Python3")\] The [Pep 3115](https://www.python.org/dev/peps/pep-3115/) defines the changes to python 3 metaclasses. In python3 you have a method \_\_prepare\_\_ that is called in the metaclass to create a dictionary or other class to store the class members.[\[1\]](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#cite_note-1) Then there is the \_\_new\_\_ method that is called to create new instances of that class. [\[2\]](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#cite_note-2) ### The type Metaclass \[[edit](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&veaction=edit&section=2 "Edit section: The type Metaclass") \\| [edit source](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=edit&section=2 "Edit section's source code: The type Metaclass")\] The metaclass for all standard Python types is the "type" object. ``` >>> type(object) <type 'type'> >>> type(int) <type 'type'> >>> type(list) <type 'type'> ``` Just like list, int and object, "type" is itself a normal Python object, and is itself an instance of a class. In this case, it is in fact an instance of itself. ``` >>> type(type) <type 'type'> ``` It can be instantiated to create new class objects similarly to the class factory example above by passing the name of the new class, the base classes to inherit from, and a dictionary defining the namespace to use. For instance, the code: ``` >>> class MyClass(BaseClass): ... attribute = 42 ``` Could also be written as: ``` >>> MyClass = type("MyClass", (BaseClass,), {'attribute' : 42}) ``` ### Metaclasses \[[edit](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&veaction=edit&section=3 "Edit section: Metaclasses") \\| [edit source](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=edit&section=3 "Edit section's source code: Metaclasses")\] It is possible to create a class with a different metaclass than type by setting the metaclass keyword argument when defining the class. When this is done, the class, and its subclass will be created using your custom metaclass. For example ``` class CustomMetaclass(type): def __init__(cls, name, bases, dct): print("Creating class %s using CustomMetaclass" % name) super(CustomMetaclass, cls).__init__(name, bases, dct) class BaseClass(metaclass=CustomMetaclass): pass class Subclass1(BaseClass): pass ``` This will print ``` Creating class BaseClass using CustomMetaclass Creating class Subclass1 using CustomMetaclass ``` By creating a custom metaclass in this way, it is possible to change how the class is constructed. This allows you to add or remove attributes and methods, register creation of classes and subclasses creation and various other manipulations when the class is created. ### More resources \[[edit](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&veaction=edit&section=4 "Edit section: More resources") \\| [edit source](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=edit&section=4 "Edit section's source code: More resources")\] - [Wikipedia article on Aspect Oriented Programming](https://en.wikipedia.org/wiki/Aspect-oriented_programming "w:Aspect-oriented programming") - [Unifying types and classes in Python 2.2](https://www.python.org/2.2/descrintro.html) - [O'Reilly Article on Python Metaclasses](http://www.onlamp.com/pub/a/python/2003/04/17/metaclasses.html) ### References \[[edit](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&veaction=edit&section=5 "Edit section: References") \\| [edit source](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&action=edit&section=5 "Edit section's source code: References")\] 1. [↑](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#cite_ref-1 "Jump up") [http://www.python.org/dev/peps/pep-3115/](https://www.python.org/dev/peps/pep-3115/) 2. [↑](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#cite_ref-2 "Jump up") <http://eli.thegreenplace.net/2011/08/14/python-metaclasses-by-example/> \| \| \| \|---\|---\| \| ![Clipboard](https://upload.wikimedia.org/wikipedia/commons/thumb/1/1f/Clipboard.svg/60px-Clipboard.svg.png) \| To do: \[Incomplete\] (see Putting Metaclasses to Work, Ira R. Forman, Scott H. Danforth?) \| \| \| \| \| \|---\|---\|---\| \| [Previous: Reflection](https://en.wikibooks.org/wiki/Python_Programming/Reflection "Python Programming/Reflection") \| [Index](https://en.wikibooks.org/wiki/Python_Programming "Python Programming") \| [Next: Tips and Tricks](https://en.wikibooks.org/wiki/Python_Programming/Tips_and_Tricks "Python Programming/Tips and Tricks") \| ![](https://en.wikibooks.org/wiki/Special:CentralAutoLogin/start?useformat=desktop&type=1x1&usesul3=1) Retrieved from "<https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&oldid=4070786>" [Category](https://en.wikibooks.org/wiki/Special:Categories "Special:Categories"): - [Book:Python Programming](https://en.wikibooks.org/wiki/Category:Book:Python_Programming "Category:Book:Python Programming") Hidden category: - [Wikibooks pages with to-do lists](https://en.wikibooks.org/wiki/Category:Wikibooks_pages_with_to-do_lists "Category:Wikibooks pages with to-do lists") - This page was last edited on 5 June 2022, at 08:10. - Text is available under the [Creative Commons Attribution-ShareAlike License](https://creativecommons.org/licenses/by-sa/4.0/); additional terms may apply. By using this site, you agree to the [Terms of Use](https://foundation.wikimedia.org/wiki/Special:MyLanguage/Policy:Terms_of_Use) and [Privacy Policy.](https://foundation.wikimedia.org/wiki/Special:MyLanguage/Policy:Privacy_policy) - [Privacy policy](https://foundation.wikimedia.org/wiki/Special:MyLanguage/Policy:Privacy_policy) - [About Wikibooks](https://en.wikibooks.org/wiki/Wikibooks:Welcome) - [Disclaimers](https://en.wikibooks.org/wiki/Wikibooks:General_disclaimer) - [Code of Conduct](https://foundation.wikimedia.org/wiki/Special:MyLanguage/Policy:Universal_Code_of_Conduct) - [Developers](https://developer.wikimedia.org/) - [Statistics](https://stats.wikimedia.org/#/en.wikibooks.org) - [Cookie statement](https://foundation.wikimedia.org/wiki/Special:MyLanguage/Policy:Cookie_statement) - [Mobile view](https://en.wikibooks.org/w/index.php?title=Python_Programming/Metaclasses&mobileaction=toggle_view_mobile) - [![Wikimedia Foundation](https://en.wikibooks.org/static/images/footer/wikimedia.svg)](https://www.wikimedia.org/) - [![Powered by MediaWiki](https://en.wikibooks.org/w/resources/assets/mediawiki_compact.svg)](https://www.mediawiki.org/) Search Toggle the table of contents Python Programming/Metaclasses 1 language [Add topic](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses)
Readable Markdown	From Wikibooks, open books for an open world In Python, classes are themselves objects. Just as other objects are instances of a particular class, classes themselves are instances of a metaclass. The [Pep 3115](https://www.python.org/dev/peps/pep-3115/) defines the changes to python 3 metaclasses. In python3 you have a method \_\_prepare\_\_ that is called in the metaclass to create a dictionary or other class to store the class members.[\[1\]](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#cite_note-1) Then there is the \_\_new\_\_ method that is called to create new instances of that class. [\[2\]](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#cite_note-2) The metaclass for all standard Python types is the "type" object. ``` >>> type(object) <type 'type'> >>> type(int) <type 'type'> >>> type(list) <type 'type'> ``` Just like list, int and object, "type" is itself a normal Python object, and is itself an instance of a class. In this case, it is in fact an instance of itself. ``` >>> type(type) <type 'type'> ``` It can be instantiated to create new class objects similarly to the class factory example above by passing the name of the new class, the base classes to inherit from, and a dictionary defining the namespace to use. For instance, the code: ``` >>> class MyClass(BaseClass): ... attribute = 42 ``` Could also be written as: ``` >>> MyClass = type("MyClass", (BaseClass,), {'attribute' : 42}) ``` It is possible to create a class with a different metaclass than type by setting the metaclass keyword argument when defining the class. When this is done, the class, and its subclass will be created using your custom metaclass. For example ``` class CustomMetaclass(type): def __init__(cls, name, bases, dct): print("Creating class %s using CustomMetaclass" % name) super(CustomMetaclass, cls).__init__(name, bases, dct) class BaseClass(metaclass=CustomMetaclass): pass class Subclass1(BaseClass): pass ``` This will print ``` Creating class BaseClass using CustomMetaclass Creating class Subclass1 using CustomMetaclass ``` By creating a custom metaclass in this way, it is possible to change how the class is constructed. This allows you to add or remove attributes and methods, register creation of classes and subclasses creation and various other manipulations when the class is created. - [Wikipedia article on Aspect Oriented Programming](https://en.wikipedia.org/wiki/Aspect-oriented_programming "w:Aspect-oriented programming") - [Unifying types and classes in Python 2.2](https://www.python.org/2.2/descrintro.html) - [O'Reilly Article on Python Metaclasses](http://www.onlamp.com/pub/a/python/2003/04/17/metaclasses.html) 1. [↑](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#cite_ref-1 "Jump up") [http://www.python.org/dev/peps/pep-3115/](https://www.python.org/dev/peps/pep-3115/) 2. [↑](https://en.wikibooks.org/wiki/Python_Programming/Metaclasses#cite_ref-2 "Jump up") <http://eli.thegreenplace.net/2011/08/14/python-metaclasses-by-example/> \| \| \| \|---\|---\| \| ![Clipboard](https://upload.wikimedia.org/wikipedia/commons/thumb/1/1f/Clipboard.svg/60px-Clipboard.svg.png) \| To do: \[Incomplete\] (see Putting Metaclasses to Work, Ira R. Forman, Scott H. Danforth?) \|
Shard	102 (laksa)
Root Hash	7794694764302024702
Unparsed URL	org,wikibooks!en,/wiki/Python_Programming/Metaclasses s443