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Calculated Shard: 69 (from laksa050)

2. Crawled Status Check

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curl -X POST \
  'http://laksa069.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://www.academis.eu/advanced_python/classes/metaclasses.html\')), getAhrefsUnparsedNoserviceFromURL(\'https://www.academis.eu/advanced_python/classes/metaclasses.html\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html","crawl_time":1776220015,"first_indexed_time":1717703576,"http_code":200,"src_unparsed":"eu,academis!www,\/advanced_python\/classes\/metaclasses.html s443","src_root_hash":"1753711494307466869","history_drop_reason":null,"meta_title":"Metaclasses - Advanced Python","meta_descriptions":[],"attrs_boilerpipe_text":"Metaclasses change the objects creation mechanics in Python.\nThe when Python creates an object, it calls the\n__new__()\nmethod of\nthe metaclass. The default behavior of\n__new__()\nis that it creates\nan instance and calls its\n__init__()\n. Now you could write a new\nmetaclass that instead of calling\n__init__()\ndoes something else.\nValid Use Cases for changing a metaclass are:\nwriting an ORM like\nDjango models\nor\nSQLAlchemy\nhijacking internal Python logic (e.g. like\npytest\ndoes)\nemulating JavaScript-like objects (the Prototype pattern)\nThroughout 20 years of Python programming, I have not come across a\nsingle situation where writing a metaclass was necessary. But it helps\nto understand Python on a deeper level.\nSimple Example\n¶\nExecute the example in\nmetaclasses.py\nand see what it does.\nComplex Example\n¶\nHere is a complex example for illustrative purposes.\nThe code might drive you nuts, so consider yourself warned!\nrun the code\nadmire what is happening\ntry to understand what is happening\nreturn to 1\nHere is the code:\nclass\nCrazyMonkeyPack\n(\ntype\n):\ndef\n__new__\n(\nmcs\n,\nname\n,\nbases\n,\ndict\n):\ncls\n=\ntype\n.\n__new__\n(\nmcs\n,\nname\n,\nbases\n,\ndict\n)\ndef\nwrapper\n(\n*\nargs\n):\ninstance\n=\n[]\nfor\ni\nin\nrange\n(\n1\n,\nargs\n[\n0\n]\n+\n1\n):\nmonkey\n=\ncls\n(\nf\n'monkey #\n{\ni\n}\n'\n)\n# calls __init__\nmonkey\n.\nstate\n=\n'crazy'\n# monkey-patches the state attribute\ninstance\n.\nappend\n(\nmonkey\n)\nreturn\ninstance\nreturn\nwrapper\nclass\nCrazyMonkeys\n(\nmetaclass\n=\nCrazyMonkeyPack\n):\n\"\"\"A self-expanding horde of monkeys\"\"\"\ndef\n__init__\n(\nself\n,\nname\n):\nself\n.\nname\n=\nname\ndef\n__repr__\n(\nself\n):\nreturn\nf\n\"<\n{\nself\n.\nname\n}\n(\n{\nself\n.\nstate\n}\n)>\"\nmonkeys\n=\nCrazyMonkeys\n(\n3\n)\n# calls __new__\nprint\n(\nmonkeys\n)\n# see what happens!\nWarning\nDon’t try using metaclasses at work, unless\nyou have excluded all alternatives\nyou really know what you are doing\nyou have talked to an experienced developer","attrs_markdown":"[Skip to content](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html#furo-main-content)\n\n[Advanced Python](https:\/\/www.academis.eu\/advanced_python\/index.html)\n\n[![Logo](https:\/\/www.academis.eu\/advanced_python\/_static\/academis_logo.png) Advanced Python](https:\/\/www.academis.eu\/advanced_python\/index.html)\n\n- [Revisit Python Basics](https:\/\/www.academis.eu\/advanced_python\/debugging\/README.html)\n\n- [Building a Python Package](https:\/\/www.academis.eu\/advanced_python\/python_package\/README.html)\n- [Set up a GitHub Repository](https:\/\/www.academis.eu\/advanced_python\/getting_started\/git_repo.html)\n- [Create a Folder Structure](https:\/\/www.academis.eu\/advanced_python\/getting_started\/structure.html)\n\n- [Writing a new Function](https:\/\/www.academis.eu\/advanced_python\/functions\/levels.html)\n- [Function parameters](https:\/\/www.academis.eu\/advanced_python\/functions\/function_parameters.html)\n- [Variable Scope](https:\/\/www.academis.eu\/advanced_python\/functions\/scope.html)\n- [Generators](https:\/\/www.academis.eu\/advanced_python\/functions\/generators.html)\n- [Map-Filter-Reduce](https:\/\/www.academis.eu\/advanced_python\/functions\/functools.html)\n- [Decorators](https:\/\/www.academis.eu\/advanced_python\/functions\/decorators.html)\n\n- [Collections](https:\/\/www.academis.eu\/advanced_python\/shortcuts\/collections.html)\n- [Comprehensions](https:\/\/www.academis.eu\/advanced_python\/shortcuts\/comprehensions.html)\n- [Enumerations](https:\/\/www.academis.eu\/advanced_python\/shortcuts\/enums.html)\n- [Demographics](https:\/\/www.academis.eu\/advanced_python\/challenges\/demographics.html)\n\n- [Refactoring](https:\/\/www.academis.eu\/advanced_python\/refactoring\/README.html)\n- [Structure of a Python script](https:\/\/www.academis.eu\/advanced_python\/structure\/main_block.html)\n- [Command-line Arguments](https:\/\/www.academis.eu\/advanced_python\/structure\/commandline_args.html)\n- [Modules](https:\/\/www.academis.eu\/advanced_python\/structure\/modules.html)\n- [Packages](https:\/\/www.academis.eu\/advanced_python\/structure\/modules.html#packages)\n- [Namespaces](https:\/\/www.academis.eu\/advanced_python\/structure\/namespaces.html)\n\n- [Classes](https:\/\/www.academis.eu\/advanced_python\/classes\/classes.html)\n- [Inheritance](https:\/\/www.academis.eu\/advanced_python\/classes\/inheritance.html)\n- [Object Composition](https:\/\/www.academis.eu\/advanced_python\/classes\/composition.html)\n- [Class Diagrams](https:\/\/www.academis.eu\/advanced_python\/classes\/class_diagram.html)\n- [Operator Overloading](https:\/\/www.academis.eu\/advanced_python\/classes\/operator_overloading.html)\n- [Properties, classmethods and staticmethods](https:\/\/www.academis.eu\/advanced_python\/classes\/properties.html)\n- [Abstract Base Classes](https:\/\/www.academis.eu\/advanced_python\/classes\/abc.html)\n- [Decorator Classes](https:\/\/www.academis.eu\/advanced_python\/classes\/decorator_class.html)\n- [Metaclasses](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html)\n- [OOP Principles](https:\/\/www.academis.eu\/advanced_python\/classes\/oop_principles.html)\n\n- [Static Code Checks](https:\/\/www.academis.eu\/advanced_python\/quality\/code_checks.html)\n- [Test Automation](https:\/\/www.academis.eu\/advanced_python\/testing\/README.html)\n- [Continuous Integration](https:\/\/www.academis.eu\/advanced_python\/quality\/continuous_integration.html)\n\n- [Debugging](https:\/\/www.academis.eu\/advanced_python\/error_handling\/debugging.html)\n- [Interactive Python Debugger](https:\/\/www.academis.eu\/advanced_python\/error_handling\/interactive_debugger.html)\n- [Handling Exceptions](https:\/\/www.academis.eu\/advanced_python\/error_handling\/exceptions.html)\n- [Warnings](https:\/\/www.academis.eu\/advanced_python\/error_handling\/warnings.html)\n- [Logging](https:\/\/www.academis.eu\/advanced_python\/error_handling\/logging.html)\n\n- [Measuring Performance](https:\/\/www.academis.eu\/advanced_python\/performance\/profiling.html)\n- [Concurrency](https:\/\/www.academis.eu\/advanced_python\/concurrency\/README.html)\n\n- [The Dungeon Explorer Game](https:\/\/www.academis.eu\/advanced_python\/software_engineering\/README.html)\n- [The Prototype](https:\/\/www.academis.eu\/advanced_python\/software_engineering\/prototype.html)\n- [Code Reviews](https:\/\/www.academis.eu\/advanced_python\/software_engineering\/code_review.html)\n\n- [Factorials](https:\/\/www.academis.eu\/advanced_python\/challenges\/factorial.html)\n- [Sorting Algorithms](https:\/\/www.academis.eu\/advanced_python\/challenges\/sorting.html)\n- [Dice](https:\/\/www.academis.eu\/advanced_python\/challenges\/dice\/dice.html)\n- [Tennis](https:\/\/www.academis.eu\/advanced_python\/challenges\/tennis.html)\n- [Memory](https:\/\/www.academis.eu\/advanced_python\/challenges\/memory\/memory.html)\n- [Magic Square](https:\/\/www.academis.eu\/advanced_python\/challenges\/magic_square.html)\n- [Josephus’ Problem](https:\/\/www.academis.eu\/advanced_python\/challenges\/josephus.html)\n- [Binary Search](https:\/\/www.academis.eu\/advanced_python\/challenges\/binary_search.html)\n- [Tree Traversal](https:\/\/www.academis.eu\/advanced_python\/challenges\/tree_traversal.html)\n- [Graph Traversal](https:\/\/www.academis.eu\/advanced_python\/challenges\/maze.html)\n- [Backpack Problem](https:\/\/www.academis.eu\/advanced_python\/challenges\/backpack_problem.html)\n- [Chained List](https:\/\/www.academis.eu\/advanced_python\/challenges\/chained_list.html)\n- [Traveling Salesman](https:\/\/www.academis.eu\/advanced_python\/challenges\/tsp.html)\n- [Blockchain](https:\/\/www.academis.eu\/advanced_python\/challenges\/blockchain.html)\n- [Decorators with a Metaclass](https:\/\/www.academis.eu\/advanced_python\/challenges\/metaclass.html)\n\n- [Project: Implement a Game](https:\/\/www.academis.eu\/advanced_python\/project.html)\n- [Hall of Fame](https:\/\/www.academis.eu\/advanced_python\/hall_of_fame.html)\n  - [Breakout](https:\/\/www.academis.eu\/advanced_python\/examples\/breakout.html)\n  - [Tetris](https:\/\/www.academis.eu\/advanced_python\/examples\/tetris.html)\n\n- [Links](https:\/\/www.academis.eu\/advanced_python\/links.html)\n\n[Back to top](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html)\n\n[View this page](https:\/\/github.com\/krother\/advanced_python\/blob\/master\/classes\/metaclasses.rst?plain=true \"View this page\")\n\n[Edit this page](https:\/\/github.com\/krother\/advanced_python\/edit\/master\/classes\/metaclasses.rst \"Edit this page\")\n\n# Metaclasses[¶](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html#metaclasses \"Link to this heading\")\nMetaclasses change the objects creation mechanics in Python.\n\nThe when Python creates an object, it calls the `__new__()` method of the metaclass. The default behavior of `__new__()` is that it creates an instance and calls its `__init__()`. Now you could write a new metaclass that instead of calling `__init__()` does something else.\n\nValid Use Cases for changing a metaclass are:\n\n- writing an ORM like **Django models** or **SQLAlchemy**\n- hijacking internal Python logic (e.g. like **pytest** does)\n- emulating JavaScript-like objects (the Prototype pattern)\n\nThroughout 20 years of Python programming, I have not come across a single situation where writing a metaclass was necessary. But it helps to understand Python on a deeper level.\n\n## Simple Example[¶](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html#simple-example \"Link to this heading\")\nExecute the example in [`metaclasses.py`](https:\/\/www.academis.eu\/advanced_python\/_downloads\/1faa83337d8d0736014f2889a475e37b\/metaclasses.py) and see what it does.\n\n## Complex Example[¶](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html#complex-example \"Link to this heading\")\nHere is a complex example for illustrative purposes. The code might drive you nuts, so consider yourself warned\\!\n\n1. run the code\n2. admire what is happening\n3. try to understand what is happening\n4. return to 1\n\nHere is the code:\n```\nclass CrazyMonkeyPack(type):\n\n    def __new__(mcs, name, bases, dict):\n        cls = type.__new__(mcs, name, bases, dict)\n\n        def wrapper(*args):\n            instance = []\n            for i in range(1, args[0]+1):\n                monkey = cls(f'monkey #{i}')  # calls __init__\n                monkey.state = 'crazy'  # monkey-patches the state attribute\n                instance.append(monkey)\n            return instance\n\n        return wrapper\n\n\nclass CrazyMonkeys(metaclass=CrazyMonkeyPack):\n    \"\"\"A self-expanding horde of monkeys\"\"\"\n    def __init__(self, name):\n        self.name = name\n\n    def __repr__(self):\n        return f\"<{self.name} ({self.state})>\"\n\n\nmonkeys = CrazyMonkeys(3)  # calls __new__\nprint(monkeys)             # see what happens!\n```\nWarning\n\nDon’t try using metaclasses at work, unless\n\n- you have excluded all alternatives\n- you really know what you are doing\n- you have talked to an experienced developer\n\n[Next OOP Principles](https:\/\/www.academis.eu\/advanced_python\/classes\/oop_principles.html)\n\n[Previous Decorator Classes](https:\/\/www.academis.eu\/advanced_python\/classes\/decorator_class.html)\n\nCopyright © 2024, Kristian Rother\n\nMade with [Sphinx](https:\/\/www.sphinx-doc.org\/) and [@pradyunsg](https:\/\/pradyunsg.me\/)'s [Furo](https:\/\/github.com\/pradyunsg\/furo)\n\nOn this page\n\n- [Metaclasses](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html)\n  - [Simple Example](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html#simple-example)\n  - [Complex Example](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html#complex-example)","attrs_readable_markdown":"Metaclasses change the objects creation mechanics in Python.\n\nThe when Python creates an object, it calls the `__new__()` method of the metaclass. The default behavior of `__new__()` is that it creates an instance and calls its `__init__()`. Now you could write a new metaclass that instead of calling `__init__()` does something else.\n\nValid Use Cases for changing a metaclass are:\n\n- writing an ORM like **Django models** or **SQLAlchemy**\n- hijacking internal Python logic (e.g. like **pytest** does)\n- emulating JavaScript-like objects (the Prototype pattern)\n\nThroughout 20 years of Python programming, I have not come across a single situation where writing a metaclass was necessary. But it helps to understand Python on a deeper level.\n\n## Simple Example[¶](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html#simple-example \"Link to this heading\")\nExecute the example in [`metaclasses.py`](https:\/\/www.academis.eu\/advanced_python\/_downloads\/1faa83337d8d0736014f2889a475e37b\/metaclasses.py) and see what it does.\n\n## Complex Example[¶](https:\/\/www.academis.eu\/advanced_python\/classes\/metaclasses.html#complex-example \"Link to this heading\")\nHere is a complex example for illustrative purposes. The code might drive you nuts, so consider yourself warned\\!\n\n1. run the code\n2. admire what is happening\n3. try to understand what is happening\n4. return to 1\n\nHere is the code:\n```\nclass CrazyMonkeyPack(type):\n\n    def __new__(mcs, name, bases, dict):\n        cls = type.__new__(mcs, name, bases, dict)\n\n        def wrapper(*args):\n            instance = []\n            for i in range(1, args[0]+1):\n                monkey = cls(f'monkey #{i}')  # calls __init__\n                monkey.state = 'crazy'  # monkey-patches the state attribute\n                instance.append(monkey)\n            return instance\n\n        return wrapper\n\n\nclass CrazyMonkeys(metaclass=CrazyMonkeyPack):\n    \"\"\"A self-expanding horde of monkeys\"\"\"\n    def __init__(self, name):\n        self.name = name\n\n    def __repr__(self):\n        return f\"<{self.name} ({self.state})>\"\n\n\nmonkeys = CrazyMonkeys(3)  # calls __new__\nprint(monkeys)             # see what happens!\n```\nWarning\n\nDon’t try using metaclasses at work, unless\n\n- you have excluded all alternatives\n- you really know what you are doing\n- you have talked to an experienced developer","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

1 day ago

🤖

ROBOTS SERVER UNREACHABLE

Failed to connect to robots server: Operation timed out after 2002 milliseconds with 0 bytes received

Page Info Filters

Filter	Status	Condition	Details
HTTP status	PASS	`download_http_code = 200`	HTTP 200
Age cutoff	PASS	`download_stamp > now() - 6 MONTH`	0.1 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://www.academis.eu/advanced_python/classes/metaclasses.html
Last Crawled	2026-04-15 02:26:55 (1 day ago)
First Indexed	2024-06-06 19:52:56 (1 year ago)
HTTP Status Code	200
Meta Title	Metaclasses - Advanced Python
Meta Description	null
Meta Canonical	null
Boilerpipe Text	Metaclasses change the objects creation mechanics in Python. The when Python creates an object, it calls the __new__() method of the metaclass. The default behavior of __new__() is that it creates an instance and calls its __init__() . Now you could write a new metaclass that instead of calling __init__() does something else. Valid Use Cases for changing a metaclass are: writing an ORM like Django models or SQLAlchemy hijacking internal Python logic (e.g. like pytest does) emulating JavaScript-like objects (the Prototype pattern) Throughout 20 years of Python programming, I have not come across a single situation where writing a metaclass was necessary. But it helps to understand Python on a deeper level. Simple Example ¶ Execute the example in metaclasses.py and see what it does. Complex Example ¶ Here is a complex example for illustrative purposes. The code might drive you nuts, so consider yourself warned! run the code admire what is happening try to understand what is happening return to 1 Here is the code: class CrazyMonkeyPack ( type ): def __new__ ( mcs , name , bases , dict ): cls = type . __new__ ( mcs , name , bases , dict ) def wrapper ( * args ): instance = [] for i in range ( 1 , args [ 0 ] + 1 ): monkey = cls ( f 'monkey # { i } ' ) # calls __init__ monkey . state = 'crazy' # monkey-patches the state attribute instance . append ( monkey ) return instance return wrapper class CrazyMonkeys ( metaclass = CrazyMonkeyPack ): """A self-expanding horde of monkeys""" def __init__ ( self , name ): self . name = name def __repr__ ( self ): return f "< { self . name } ( { self . state } )>" monkeys = CrazyMonkeys ( 3 ) # calls __new__ print ( monkeys ) # see what happens! Warning Don’t try using metaclasses at work, unless you have excluded all alternatives you really know what you are doing you have talked to an experienced developer
Markdown	[Skip to content](https://www.academis.eu/advanced_python/classes/metaclasses.html#furo-main-content) [Advanced Python](https://www.academis.eu/advanced_python/index.html) [![Logo](https://www.academis.eu/advanced_python/_static/academis_logo.png) Advanced Python](https://www.academis.eu/advanced_python/index.html) - [Revisit Python Basics](https://www.academis.eu/advanced_python/debugging/README.html) - [Building a Python Package](https://www.academis.eu/advanced_python/python_package/README.html) - [Set up a GitHub Repository](https://www.academis.eu/advanced_python/getting_started/git_repo.html) - [Create a Folder Structure](https://www.academis.eu/advanced_python/getting_started/structure.html) - [Writing a new Function](https://www.academis.eu/advanced_python/functions/levels.html) - [Function parameters](https://www.academis.eu/advanced_python/functions/function_parameters.html) - [Variable Scope](https://www.academis.eu/advanced_python/functions/scope.html) - [Generators](https://www.academis.eu/advanced_python/functions/generators.html) - [Map-Filter-Reduce](https://www.academis.eu/advanced_python/functions/functools.html) - [Decorators](https://www.academis.eu/advanced_python/functions/decorators.html) - [Collections](https://www.academis.eu/advanced_python/shortcuts/collections.html) - [Comprehensions](https://www.academis.eu/advanced_python/shortcuts/comprehensions.html) - [Enumerations](https://www.academis.eu/advanced_python/shortcuts/enums.html) - [Demographics](https://www.academis.eu/advanced_python/challenges/demographics.html) - [Refactoring](https://www.academis.eu/advanced_python/refactoring/README.html) - [Structure of a Python script](https://www.academis.eu/advanced_python/structure/main_block.html) - [Command-line Arguments](https://www.academis.eu/advanced_python/structure/commandline_args.html) - [Modules](https://www.academis.eu/advanced_python/structure/modules.html) - [Packages](https://www.academis.eu/advanced_python/structure/modules.html#packages) - [Namespaces](https://www.academis.eu/advanced_python/structure/namespaces.html) - 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[Traveling Salesman](https://www.academis.eu/advanced_python/challenges/tsp.html) - [Blockchain](https://www.academis.eu/advanced_python/challenges/blockchain.html) - [Decorators with a Metaclass](https://www.academis.eu/advanced_python/challenges/metaclass.html) - [Project: Implement a Game](https://www.academis.eu/advanced_python/project.html) - [Hall of Fame](https://www.academis.eu/advanced_python/hall_of_fame.html) - [Breakout](https://www.academis.eu/advanced_python/examples/breakout.html) - [Tetris](https://www.academis.eu/advanced_python/examples/tetris.html) - [Links](https://www.academis.eu/advanced_python/links.html) [Back to top](https://www.academis.eu/advanced_python/classes/metaclasses.html) [View this page](https://github.com/krother/advanced_python/blob/master/classes/metaclasses.rst?plain=true "View this page") [Edit this page](https://github.com/krother/advanced_python/edit/master/classes/metaclasses.rst "Edit this page") # Metaclasses[¶](https://www.academis.eu/advanced_python/classes/metaclasses.html#metaclasses "Link to this heading") Metaclasses change the objects creation mechanics in Python. The when Python creates an object, it calls the `__new__()` method of the metaclass. The default behavior of `__new__()` is that it creates an instance and calls its `__init__()`. Now you could write a new metaclass that instead of calling `__init__()` does something else. Valid Use Cases for changing a metaclass are: - writing an ORM like Django models or SQLAlchemy - hijacking internal Python logic (e.g. like pytest does) - emulating JavaScript-like objects (the Prototype pattern) Throughout 20 years of Python programming, I have not come across a single situation where writing a metaclass was necessary. But it helps to understand Python on a deeper level. ## Simple Example[¶](https://www.academis.eu/advanced_python/classes/metaclasses.html#simple-example "Link to this heading") Execute the example in [`metaclasses.py`](https://www.academis.eu/advanced_python/_downloads/1faa83337d8d0736014f2889a475e37b/metaclasses.py) and see what it does. ## Complex Example[¶](https://www.academis.eu/advanced_python/classes/metaclasses.html#complex-example "Link to this heading") Here is a complex example for illustrative purposes. The code might drive you nuts, so consider yourself warned\! 1. run the code 2. admire what is happening 3. try to understand what is happening 4. return to 1 Here is the code: ``` class CrazyMonkeyPack(type): def __new__(mcs, name, bases, dict): cls = type.__new__(mcs, name, bases, dict) def wrapper(*args): instance = [] for i in range(1, args[0]+1): monkey = cls(f'monkey #{i}') # calls __init__ monkey.state = 'crazy' # monkey-patches the state attribute instance.append(monkey) return instance return wrapper class CrazyMonkeys(metaclass=CrazyMonkeyPack): """A self-expanding horde of monkeys""" def __init__(self, name): self.name = name def __repr__(self): return f"<{self.name} ({self.state})>" monkeys = CrazyMonkeys(3) # calls __new__ print(monkeys) # see what happens! ``` Warning Don’t try using metaclasses at work, unless - you have excluded all alternatives - you really know what you are doing - you have talked to an experienced developer [Next OOP Principles](https://www.academis.eu/advanced_python/classes/oop_principles.html) [Previous Decorator Classes](https://www.academis.eu/advanced_python/classes/decorator_class.html) Copyright © 2024, Kristian Rother Made with [Sphinx](https://www.sphinx-doc.org/) and [@pradyunsg](https://pradyunsg.me/)'s [Furo](https://github.com/pradyunsg/furo) On this page - [Metaclasses](https://www.academis.eu/advanced_python/classes/metaclasses.html) - [Simple Example](https://www.academis.eu/advanced_python/classes/metaclasses.html#simple-example) - [Complex Example](https://www.academis.eu/advanced_python/classes/metaclasses.html#complex-example)
Readable Markdown	Metaclasses change the objects creation mechanics in Python. The when Python creates an object, it calls the `__new__()` method of the metaclass. The default behavior of `__new__()` is that it creates an instance and calls its `__init__()`. Now you could write a new metaclass that instead of calling `__init__()` does something else. Valid Use Cases for changing a metaclass are: - writing an ORM like Django models or SQLAlchemy - hijacking internal Python logic (e.g. like pytest does) - emulating JavaScript-like objects (the Prototype pattern) Throughout 20 years of Python programming, I have not come across a single situation where writing a metaclass was necessary. But it helps to understand Python on a deeper level. ## Simple Example[¶](https://www.academis.eu/advanced_python/classes/metaclasses.html#simple-example "Link to this heading") Execute the example in [`metaclasses.py`](https://www.academis.eu/advanced_python/_downloads/1faa83337d8d0736014f2889a475e37b/metaclasses.py) and see what it does. ## Complex Example[¶](https://www.academis.eu/advanced_python/classes/metaclasses.html#complex-example "Link to this heading") Here is a complex example for illustrative purposes. The code might drive you nuts, so consider yourself warned\! 1. run the code 2. admire what is happening 3. try to understand what is happening 4. return to 1 Here is the code: ``` class CrazyMonkeyPack(type): def __new__(mcs, name, bases, dict): cls = type.__new__(mcs, name, bases, dict) def wrapper(*args): instance = [] for i in range(1, args[0]+1): monkey = cls(f'monkey #{i}') # calls __init__ monkey.state = 'crazy' # monkey-patches the state attribute instance.append(monkey) return instance return wrapper class CrazyMonkeys(metaclass=CrazyMonkeyPack): """A self-expanding horde of monkeys""" def __init__(self, name): self.name = name def __repr__(self): return f"<{self.name} ({self.state})>" monkeys = CrazyMonkeys(3) # calls __new__ print(monkeys) # see what happens! ``` Warning Don’t try using metaclasses at work, unless - you have excluded all alternatives - you really know what you are doing - you have talked to an experienced developer
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