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Calculated Shard: 54 (from laksa142)

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curl -X POST \
  'http://laksa054.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, ml_categories_json AS ml_categories_json, ml_types_json AS ml_types_json, ml_intent_types_json AS ml_intent_types_json, meta_language AS meta_language, attrs_author AS attrs_author, ifNull(toUnixTimestamp(attrs_publish_time), 0) AS attrs_publish_time, ifNull(toUnixTimestamp(attrs_original_publish_time), 0) AS attrs_original_publish_time, ifNull(attrs_is_republished, 0) AS attrs_is_republished, ifNull(attrs_nr_words, 0) AS attrs_nr_words, ifNull(attrs_boilerpipe_nr_words, 0) AS attrs_boilerpipe_nr_words, ifNull(body_ext_links_number, 0) AS body_ext_links_number, ifNull(body_int_links_number, 0) AS body_int_links_number, ifNull(meta_nofollow, 0) AS meta_nofollow, ifNull(meta_noarchive, 0) AS meta_noarchive, ifNull(props_was_rendered, 0) AS props_was_rendered, ifNull(src_redirect, \'\') AS src_redirect, ifNull(download_time_msec, 0) AS download_time_msec, ifNull(download_ttfb_msec, 0) AS download_ttfb_msec, ifNull(download_size, 0) AS download_size FROM crawler3.page_info_local FINAL PREWHERE (src_root_hash, src_unparsed) IN ((getAhrefsRootHashFromUnparsed(getAhrefsUnparsedNoserviceFromURL(\'https://www.python-engineer.com/posts/metaclasses-python/\')), getAhrefsUnparsedNoserviceFromURL(\'https://www.python-engineer.com/posts/metaclasses-python/\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/","crawl_time":1776282141,"first_indexed_time":1669503121,"http_code":200,"src_unparsed":"com,python-engineer!www,\/posts\/metaclasses-python\/ s443","src_root_hash":"10843413975543007054","history_drop_reason":null,"meta_title":"What are metaclasses in Python - Python Engineer","meta_descriptions":["A beginners guide to metaclasses in Python."],"attrs_boilerpipe_text":"A beginners guide to metaclasses in Python.\nPython\nBasics\nMetaclass is an advanced Python concept and is not used in day-to-day programming tasks. They allow a user to precisely define a class state, change its attributes and control method behavior.\nIn this article, we will understand how metaclasses can be created and used in Python.\n1. Type and OOP\n¶\nEverything is an object in Python, including classes. Hence, if classes are an object, they must be created by another class also called as Metaclass.\nSo, a metaclass is just another class that creates class objects. Usually,\ntype\nis the built-in metaclass Python uses for every class. But we can also create our own metaclasses with custom behavior.\n2. Understanding the\ntype()\nfunction\n¶\nTo fully understand Metaclasses, one needs to thoroughly understand\ntype()\n.\nIn Python,\ntype()\nis used to identify the data type of a variable. If we have the following code\nvar\n=\n12\nprint\n(\ntype\n(\nvar\n))\nConsole output would be:\n<class 'int'>\nIndicating\nvar\nis an instance of the class\nint\n.\nIn the case of a user-defined class, we get the following output.\nclass\nFoo\n:\ndef\nbar\n():\npass\nfoo_obj\n=\nFoo\n()\nprint\n(\n\"Type of Foo's instance is:\"\n,\ntype\n(\nfoo_obj\n))\nprint\n(\n\"Type of Foo is:\"\n,\ntype\n(\nFoo\n))\nType of Foo's instance is: <class '__main__.Foo'>\nType of Foo is: <class 'type'>\nHere, we can see\nfoo_obj\nis correctly identified as an instance of class\nFoo\n, but the type of class\nFoo\nindicates that it is an instance of class\ntype\n, establishing the point that classes are indeed created by other classes.\nCreating classes dynamically using\ntype()\n¶\nThis is one more concept we should know before diving into metaclasses.\nThis usage of\ntype()\nis fairly unknown. The\ntype()\nfunction can take three arguments and can be used to create classes dynamically:\ntype\n(\nname\n:\nstr\n,\nbases\n:\ntuple\n,\nattributes\n:\ndict\n)\nname\nindicates the name of a class\nbases\nis a tuple of classes to be inherited\nattributes\nis a dictionary including all variables and methods to be declared inside the new class\nExample:\nDummy\n=\ntype\n(\n\"Dummy\"\n,\n(),\n{\n\"var1\"\n:\n\"lorem ipsum sit domat\"\n})\ndummy\n=\nDummy\n()\nprint\n(\ndummy\n.\nvar1\n)\nOutput:\nlorem ipsum sit domat\nThe main purpose of a metaclass is to change the class automatically when it's created.\nTo use class as a metaclass we pass\nmetaclass=ClassName\nas a class parameter. The metaclass itself is implemented like a normal class with any behavior we want. Usually the\n__new__\nfunction is implemented.\nThe name of the metaclass should ideally make the intent of the class clear:\nExample:\nclass\nPrintAttributeMeta\n:\ndef\n__new__\n(\ncls\n,\nclass_name\n,\nbases\n,\nattributes\n):\nprint\n(\n\"Attributes received in PrintAttributeMeta:\"\n,\nattributes\n)\nreturn\ntype\n(\nclass_name\n,\nbases\n,\nattributes\n)\nclass\nDummy\n(\nmetaclass\n=\nPrintAttributeMeta\n):\nclass_var1\n=\n\"lorem\"\nclass_var2\n=\n45\nOutput:\nAttributes received in PrintAttributeMeta: {'__module__': '__main__', '__qualname__': 'Dummy', 'class_var1': 'lorem', 'class_var2': 45}\nThis meta class simply prints all attributes, and then creates and returns a new class by using the\ntype()\nfunction.\nNote that there’s output without creating an instance of the class because the\n__new__()\nmethod is called when the class is created.\nUsing Metaclasses, the user can perform multitude of validations and modification such as changing variable names, raising error on receiving an invalid data type, checking inheritance levels, etc.\nFor this example, we will write a Metaclass that raises a\nValueError\nwhen a negative value is received in\nattributes\n.\nclass\nPositiveNumbersMeta\n:\ndef\n__new__\n(\ncls\n,\nclass_name\n,\nbases\n,\nattributes\n):\nfor\nattr\n,\nvalue\nin\nattributes\n.\nitems\n():\nif\nisinstance\n(\nvalue\n,\n(\nint\n,\nfloat\n))\nand\nvalue\n<\n0\n:\nraise\nValueError\n(\nf\n\"Negative values are not allowed in\n{\nclass_name\n}\n\"\n)\nreturn\ntype\n(\nclass_name\n,\nbases\n,\nattributes\n)\nclass\nMyClass\n(\nmetaclass\n=\nPositiveNumbersMeta\n):\nclass_var1\n=\n23\nclass_var2\n=\n-\n1\nOutput:\nTraceback (most recent call last):\nFile \"<stdin>\", line 1, in <module>\nFile \"<stdin>\", line 5, in __new__\nValueError: Negative values are not allowed in MyClass\nWe could also change attributes:\nclass\nPositiveNumbersMeta\n:\ndef\n__new__\n(\ncls\n,\nclass_name\n,\nbases\n,\nattributes\n):\nfor\nattr\n,\nvalue\nin\nattributes\n.\nitems\n():\nif\nisinstance\n(\nvalue\n,\n(\nint\n,\nfloat\n))\nand\nvalue\n<\n0\n:\nattributes\n[\nattr\n]\n=\n-\nvalue\nreturn\ntype\n(\nclass_name\n,\nbases\n,\nattributes\n)\nclass\nMyClass\n(\nmetaclass\n=\nPositiveNumbersMeta\n):\nclass_var1\n=\n23\nclass_var2\n=\n-\n1\na\n=\nMyClass\n()\nprint\n(\na\n.\nclass_var2\n)\n# 1\n5. Use proper OOP principles\n¶\nIn the previous example we are calling\ntype\ndirectly and aren't overriding or calling the parent's\n__new__\n. Let's do that instead:\nclass\nPositiveNumbersMeta\n(\ntype\n):\ndef\n__new__\n(\ncls\n,\nclass_name\n,\nbases\n,\nattributes\n):\nfor\nattr\n,\nvalue\nin\nattributes\n.\nitems\n():\nif\nisinstance\n(\nvalue\n,\n(\nint\n,\nfloat\n))\nand\nvalue\n<\n0\n:\nattributes\n[\nattr\n]\n=\n-\nvalue\nreturn\nsuper\n.\n__new__\n(\nclass_name\n,\nbases\n,\nattributes\n)\nNow our metaclass inherits from\ntype\n, and by calling the\nsuper.__new__()\nwe are using the OOP syntax and call the\n__new__()\nfunction of the base class.\nThere are several reasons to do so:\nThe intention is clear. When you read PositiveNumbersMeta, you know what's going to follow\nYou can use OOP. Metaclass can inherit from metaclass, override parent methods, etc. Metaclasses can even use other metaclasses.\nYou can structure your code better. You never use metaclasses for something as trivial as the above example. It's usually for something complicated. Having the ability to make several methods and group them in one class is very useful to make the code easier to read.\nYou can hook on\n__new__\n,\n__init__\nand\n__call__\n. Which will allow you to do different stuff, Even if usually you can do it all in\n__new__\n, some people are just more comfortable using\n__init__\n.\nResource: Here's a great\nStackOverflow answer\nthat explains it well.\nNote that Metaclasses are usually only used in more complex scenarios, and in 99% of cases you don't have to worry about it. One common use case is when creating an API. A typical example of this is the\nDjango ORM\n.\nFREE VS Code \/ PyCharm Extensions I Use\n✅ Write cleaner code with Sourcery, instant refactoring suggestions:\nLink*\nPython Problem-Solving Bootcamp\n🚀 Solve 42 programming puzzles over the course of 21 days:\nLink*\n* These are affiliate link. By clicking on it you will not have any additional costs. Instead, you will support my project. Thank you! 🙏","attrs_markdown":"[Skip to content](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#1-type-and-oop)\n\n[![logo](https:\/\/www.python-engineer.com\/img\/logo.png)](https:\/\/www.python-engineer.com\/ \"Python Engineer\")\n\nPython Engineer\n\nWhat are metaclasses in Python\n\nInitializing search\n\n- [Home](https:\/\/www.python-engineer.com\/)\n- [Posts](https:\/\/www.python-engineer.com\/posts\/)\n- [About](https:\/\/www.python-engineer.com\/about\/)\n- [Newsletter](https:\/\/www.python-engineer.com\/newsletter\/)\n\n[![logo](https:\/\/www.python-engineer.com\/img\/logo.png)](https:\/\/www.python-engineer.com\/ \"Python Engineer\")  Python Engineer\n\n- [Home](https:\/\/www.python-engineer.com\/)\n- [Posts](https:\/\/www.python-engineer.com\/posts\/)\n  Posts\n  \n  - Tutorials\n    Tutorials\n    \n    - [How to split a List into equally sized chunks in Python](https:\/\/www.python-engineer.com\/posts\/split_list_in_chunks\/)\n    - [How to delete a key from a dictionary in Python](https:\/\/www.python-engineer.com\/posts\/delete-key-dictionary\/)\n    - [How to convert a Google Colab to Markdown](https:\/\/www.python-engineer.com\/posts\/convert-colab-markdown\/)\n    - [LangChain Tutorial in Python - Crash Course](https:\/\/www.python-engineer.com\/posts\/langchain-crash-course\/)\n    - [How to write your own context manager in Python](https:\/\/www.python-engineer.com\/posts\/contextlib-contextmanager\/)\n    - [How to easily remove the background of images in Python](https:\/\/www.python-engineer.com\/posts\/remove_background\/)\n    - [How to work with the Notion API in Python](https:\/\/www.python-engineer.com\/posts\/notion-api-python\/)\n    - [How to measure the elapsed time in Python](https:\/\/www.python-engineer.com\/posts\/measure-elapsed-time\/)\n    - [How to copy a List in Python without side effects](https:\/\/www.python-engineer.com\/posts\/how-to-copy-list\/)\n    - [How to check if a List is empty in Python](https:\/\/www.python-engineer.com\/posts\/check-if-list-is-empty\/)\n    - [How to sort a dictionary by values in Python](https:\/\/www.python-engineer.com\/posts\/sort-dictionary-values\/)\n    - 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[1\\. Type and OOP](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#1-type-and-oop)\n      - [2\\. Understanding the type() function](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#2-understanding-the-type-function)\n      - [Creating classes dynamically using type()](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#creating-classes-dynamically-using-type)\n      - [3\\. Creating Metaclass](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#3-creating-metaclass)\n      - [4\\. Modifying a class state using Metaclass](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#4-modifying-a-class-state-using-metaclass)\n      - [5\\. Use proper OOP principles](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#5-use-proper-oop-principles)\n      - [6\\. When to use metaclasses?](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#6-when-to-use-metaclasses)\n    - [How to randomly select an item from a list?](https:\/\/www.python-engineer.com\/posts\/randomly-select-item\/)\n    - [Getting Started with OpenCV in Python](https:\/\/www.python-engineer.com\/posts\/opencv-python-intro\/)\n    - [What are global, local, and nonlocal scopes in Python](https:\/\/www.python-engineer.com\/posts\/global-local-nonlocal-python\/)\n    - [What is self in Python classes](https:\/\/www.python-engineer.com\/posts\/self-python\/)\n    - [Create a Task Tracker App for the Terminal with Python (Rich, Typer, Sqlite3)](https:\/\/www.python-engineer.com\/posts\/task-tracker-cli\/)\n    - [Introduction to Graph Machine Learning](https:\/\/www.python-engineer.com\/posts\/graph-ml-intro\/)\n    - [How to check if an object is iterable in Python](https:\/\/www.python-engineer.com\/posts\/object-is-iterable\/)\n    - [How to slice sequences in Python](https:\/\/www.python-engineer.com\/posts\/sequence-slicing\/)\n    - 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[Tags](https:\/\/www.python-engineer.com\/tags\/)\n  - Authors\n    Authors\n    \n    - [Authors](https:\/\/www.python-engineer.com\/authors\/)\n- [About](https:\/\/www.python-engineer.com\/about\/)\n- [Newsletter](https:\/\/www.python-engineer.com\/newsletter\/)\n\nOn this page\n\n- [1\\. Type and OOP](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#1-type-and-oop)\n- [2\\. Understanding the type() function](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#2-understanding-the-type-function)\n- [Creating classes dynamically using type()](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#creating-classes-dynamically-using-type)\n- [3\\. Creating Metaclass](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#3-creating-metaclass)\n- [4\\. Modifying a class state using Metaclass](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#4-modifying-a-class-state-using-metaclass)\n- [5\\. Use proper OOP principles](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#5-use-proper-oop-principles)\n- [6\\. When to use metaclasses?](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#6-when-to-use-metaclasses)\n\n# What are metaclasses in Python\n**A beginners guide to metaclasses in Python.**\n\n![Pratik Choudhari](https:\/\/www.python-engineer.com\/img\/pratik.webp)\n\n**Pratik Choudhari** · · · February 07, 2022 · 9 min read\n\n[Python](https:\/\/www.python-engineer.com\/tags\/#python) [Basics](https:\/\/www.python-engineer.com\/tags\/#basics)\n***\nMetaclass is an advanced Python concept and is not used in day-to-day programming tasks. They allow a user to precisely define a class state, change its attributes and control method behavior.\n\n\\#more\n\nIn this article, we will understand how metaclasses can be created and used in Python.\n\n## 1\\. Type and OOP[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#1-type-and-oop \"Permanent link\")\nEverything is an object in Python, including classes. Hence, if classes are an object, they must be created by another class also called as Metaclass.\n\n![class hierarchy](https:\/\/www.python-engineer.com\/img\/2022-01-29-metaclasses\/metaclass.webp)\n\nSo, a metaclass is just another class that creates class objects. Usually, `type` is the built-in metaclass Python uses for every class. But we can also create our own metaclasses with custom behavior.\n\n## 2\\. Understanding the `type()` function[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#2-understanding-the-type-function \"Permanent link\")\nTo fully understand Metaclasses, one needs to thoroughly understand `type()`.\n\nIn Python, `type()` is used to identify the data type of a variable. If we have the following code\n```\n\n```\nConsole output would be:\n```\n<class 'int'>\n```\nIndicating `var` is an instance of the class `int`.\n\nIn the case of a user-defined class, we get the following output.\n```\n\n```\n```\n\n```\nHere, we can see `foo_obj` is correctly identified as an instance of class `Foo`, but the type of class `Foo` indicates that it is an instance of class `type`, establishing the point that classes are indeed created by other classes.\n\n## Creating classes dynamically using `type()`[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#creating-classes-dynamically-using-type \"Permanent link\")\nThis is one more concept we should know before diving into metaclasses.\n\nThis usage of `type()` is fairly unknown. The `type()` function can take three arguments and can be used to create classes dynamically:\n```\ntype(name: str, bases: tuple, attributes: dict)\n```\n- `name` indicates the name of a class\n- `bases` is a tuple of classes to be inherited\n- `attributes` is a dictionary including all variables and methods to be declared inside the new class\n\nExample:\n```\n\n```\nOutput:\n```\nlorem ipsum sit domat\n```\n## 3\\. Creating Metaclass[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#3-creating-metaclass \"Permanent link\")\nThe main purpose of a metaclass is to change the class automatically when it's created.\n\nTo use class as a metaclass we pass `metaclass=ClassName` as a class parameter. The metaclass itself is implemented like a normal class with any behavior we want. Usually the `__new__` function is implemented.\n\nThe name of the metaclass should ideally make the intent of the class clear:\n\nExample:\n```\n\n```\nOutput:\n```\nAttributes received in PrintAttributeMeta: {'__module__': '__main__', '__qualname__': 'Dummy', 'class_var1': 'lorem', 'class_var2': 45}\n```\nThis meta class simply prints all attributes, and then creates and returns a new class by using the `type()` function.\n\nNote that there’s output without creating an instance of the class because the `__new__()` method is called when the class is created.\n\n## 4\\. Modifying a class state using Metaclass[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#4-modifying-a-class-state-using-metaclass \"Permanent link\")\nUsing Metaclasses, the user can perform multitude of validations and modification such as changing variable names, raising error on receiving an invalid data type, checking inheritance levels, etc.\n\nFor this example, we will write a Metaclass that raises a `ValueError` when a negative value is received in `attributes`.\n```\n\n```\nOutput:\n```\n\n```\nWe could also change attributes:\n```\n\n```\n## 5\\. Use proper OOP principles[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#5-use-proper-oop-principles \"Permanent link\")\nIn the previous example we are calling `type` directly and aren't overriding or calling the parent's `__new__`. Let's do that instead:\n```\n\n```\nNow our metaclass inherits from `type`, and by calling the `super.__new__()` we are using the OOP syntax and call the `__new__()` function of the base class.\n\n## 6\\. When to use metaclasses?[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#6-when-to-use-metaclasses \"Permanent link\")\nThere are several reasons to do so:\n\n- The intention is clear. When you read PositiveNumbersMeta, you know what's going to follow\n- You can use OOP. Metaclass can inherit from metaclass, override parent methods, etc. Metaclasses can even use other metaclasses.\n- You can structure your code better. You never use metaclasses for something as trivial as the above example. It's usually for something complicated. Having the ability to make several methods and group them in one class is very useful to make the code easier to read.\n- You can hook on `__new__`, `__init__` and `__call__`. Which will allow you to do different stuff, Even if usually you can do it all in `__new__`, some people are just more comfortable using `__init__`.\n\nResource: Here's a great [StackOverflow answer](https:\/\/stackoverflow.com\/questions\/100003\/what-are-metaclasses-in-python) that explains it well.\n\nNote that Metaclasses are usually only used in more complex scenarios, and in 99% of cases you don't have to worry about it. One common use case is when creating an API. A typical example of this is the [Django ORM](https:\/\/docs.djangoproject.com\/en\/4.0\/topics\/db\/models\/).\n***\nFREE VS Code \/ PyCharm Extensions I Use\n\n✅ Write cleaner code with Sourcery, instant refactoring suggestions: [Link\\*](https:\/\/sourcery.ai\/?utm_source=youtube&utm_campaign=pythonengineer)\n***\nPython Problem-Solving Bootcamp\n\n🚀 Solve 42 programming puzzles over the course of 21 days: [Link\\*](https:\/\/www.python-engineer.com\/go\/sponsor-pythonbootcamp)\n\n\\* These are affiliate link. By clicking on it you will not have any additional costs. Instead, you will support my project. Thank you! 🙏\n***\n\n[Previous  Working with Images in OpenCV using Python](https:\/\/www.python-engineer.com\/posts\/opencv-images\/)\n\n[Next  How to randomly select an item from a list?](https:\/\/www.python-engineer.com\/posts\/randomly-select-item\/)\n\nCopyright © 2023 Patrick Loeber\n\nMade with  [Material for MkDocs](https:\/\/squidfunk.github.io\/mkdocs-material\/)\n\n[Privacy Policy](https:\/\/www.python-engineer.com\/privacy-policy\/) · [Legal Notice](https:\/\/www.python-engineer.com\/legal-notice\/)","attrs_readable_markdown":"**A beginners guide to metaclasses in Python.**\n\n[Python](https:\/\/www.python-engineer.com\/tags\/#python) [Basics](https:\/\/www.python-engineer.com\/tags\/#basics)\n***\nMetaclass is an advanced Python concept and is not used in day-to-day programming tasks. They allow a user to precisely define a class state, change its attributes and control method behavior.\n\nIn this article, we will understand how metaclasses can be created and used in Python.\n\n## 1\\. Type and OOP[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#1-type-and-oop \"Permanent link\")\nEverything is an object in Python, including classes. Hence, if classes are an object, they must be created by another class also called as Metaclass.\n\n![class hierarchy](https:\/\/www.python-engineer.com\/img\/2022-01-29-metaclasses\/metaclass.webp)\n\nSo, a metaclass is just another class that creates class objects. Usually, `type` is the built-in metaclass Python uses for every class. But we can also create our own metaclasses with custom behavior.\n\n## 2\\. Understanding the `type()` function[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#2-understanding-the-type-function \"Permanent link\")\nTo fully understand Metaclasses, one needs to thoroughly understand `type()`.\n\nIn Python, `type()` is used to identify the data type of a variable. If we have the following code\n```\n\n```\nConsole output would be:\n```\n<class 'int'>\n```\nIndicating `var` is an instance of the class `int`.\n\nIn the case of a user-defined class, we get the following output.\n```\n\n```\n```\n\n```\nHere, we can see `foo_obj` is correctly identified as an instance of class `Foo`, but the type of class `Foo` indicates that it is an instance of class `type`, establishing the point that classes are indeed created by other classes.\n\n## Creating classes dynamically using `type()`[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#creating-classes-dynamically-using-type \"Permanent link\")\nThis is one more concept we should know before diving into metaclasses.\n\nThis usage of `type()` is fairly unknown. The `type()` function can take three arguments and can be used to create classes dynamically:\n```\ntype(name: str, bases: tuple, attributes: dict)\n```\n- `name` indicates the name of a class\n- `bases` is a tuple of classes to be inherited\n- `attributes` is a dictionary including all variables and methods to be declared inside the new class\n\nExample:\n```\n\n```\nOutput:\n```\nlorem ipsum sit domat\n```\nThe main purpose of a metaclass is to change the class automatically when it's created.\n\nTo use class as a metaclass we pass `metaclass=ClassName` as a class parameter. The metaclass itself is implemented like a normal class with any behavior we want. Usually the `__new__` function is implemented.\n\nThe name of the metaclass should ideally make the intent of the class clear:\n\nExample:\n```\n\n```\nOutput:\n```\nAttributes received in PrintAttributeMeta: {'__module__': '__main__', '__qualname__': 'Dummy', 'class_var1': 'lorem', 'class_var2': 45}\n```\nThis meta class simply prints all attributes, and then creates and returns a new class by using the `type()` function.\n\nNote that there’s output without creating an instance of the class because the `__new__()` method is called when the class is created.\n\nUsing Metaclasses, the user can perform multitude of validations and modification such as changing variable names, raising error on receiving an invalid data type, checking inheritance levels, etc.\n\nFor this example, we will write a Metaclass that raises a `ValueError` when a negative value is received in `attributes`.\n```\n\n```\nOutput:\n```\n\n```\nWe could also change attributes:\n```\n\n```\n## 5\\. Use proper OOP principles[¶](https:\/\/www.python-engineer.com\/posts\/metaclasses-python\/#5-use-proper-oop-principles \"Permanent link\")\nIn the previous example we are calling `type` directly and aren't overriding or calling the parent's `__new__`. Let's do that instead:\n```\n\n```\nNow our metaclass inherits from `type`, and by calling the `super.__new__()` we are using the OOP syntax and call the `__new__()` function of the base class.\n\nThere are several reasons to do so:\n\n- The intention is clear. When you read PositiveNumbersMeta, you know what's going to follow\n- You can use OOP. Metaclass can inherit from metaclass, override parent methods, etc. Metaclasses can even use other metaclasses.\n- You can structure your code better. You never use metaclasses for something as trivial as the above example. It's usually for something complicated. Having the ability to make several methods and group them in one class is very useful to make the code easier to read.\n- You can hook on `__new__`, `__init__` and `__call__`. Which will allow you to do different stuff, Even if usually you can do it all in `__new__`, some people are just more comfortable using `__init__`.\n\nResource: Here's a great [StackOverflow answer](https:\/\/stackoverflow.com\/questions\/100003\/what-are-metaclasses-in-python) that explains it well.\n\nNote that Metaclasses are usually only used in more complex scenarios, and in 99% of cases you don't have to worry about it. One common use case is when creating an API. A typical example of this is the [Django ORM](https:\/\/docs.djangoproject.com\/en\/4.0\/topics\/db\/models\/).\n***\nFREE VS Code \/ PyCharm Extensions I Use\n\n✅ Write cleaner code with Sourcery, instant refactoring suggestions: [Link\\*](https:\/\/sourcery.ai\/?utm_source=youtube&utm_campaign=pythonengineer)\n***\nPython Problem-Solving Bootcamp\n\n🚀 Solve 42 programming puzzles over the course of 21 days: [Link\\*](https:\/\/www.python-engineer.com\/go\/sponsor-pythonbootcamp)\n\n\\* These are affiliate link. By clicking on it you will not have any additional costs. Instead, you will support my project. 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Boilerpipe Text	A beginners guide to metaclasses in Python. Python Basics Metaclass is an advanced Python concept and is not used in day-to-day programming tasks. They allow a user to precisely define a class state, change its attributes and control method behavior. In this article, we will understand how metaclasses can be created and used in Python. 1. Type and OOP ¶ Everything is an object in Python, including classes. Hence, if classes are an object, they must be created by another class also called as Metaclass. So, a metaclass is just another class that creates class objects. Usually, type is the built-in metaclass Python uses for every class. But we can also create our own metaclasses with custom behavior. 2. Understanding the type() function ¶ To fully understand Metaclasses, one needs to thoroughly understand type() . In Python, type() is used to identify the data type of a variable. If we have the following code var = 12 print ( type ( var )) Console output would be: <class 'int'> Indicating var is an instance of the class int . In the case of a user-defined class, we get the following output. class Foo : def bar (): pass foo_obj = Foo () print ( "Type of Foo's instance is:" , type ( foo_obj )) print ( "Type of Foo is:" , type ( Foo )) Type of Foo's instance is: <class '__main__.Foo'> Type of Foo is: <class 'type'> Here, we can see foo_obj is correctly identified as an instance of class Foo , but the type of class Foo indicates that it is an instance of class type , establishing the point that classes are indeed created by other classes. Creating classes dynamically using type() ¶ This is one more concept we should know before diving into metaclasses. This usage of type() is fairly unknown. The type() function can take three arguments and can be used to create classes dynamically: type ( name : str , bases : tuple , attributes : dict ) name indicates the name of a class bases is a tuple of classes to be inherited attributes is a dictionary including all variables and methods to be declared inside the new class Example: Dummy = type ( "Dummy" , (), { "var1" : "lorem ipsum sit domat" }) dummy = Dummy () print ( dummy . var1 ) Output: lorem ipsum sit domat The main purpose of a metaclass is to change the class automatically when it's created. To use class as a metaclass we pass metaclass=ClassName as a class parameter. The metaclass itself is implemented like a normal class with any behavior we want. Usually the __new__ function is implemented. The name of the metaclass should ideally make the intent of the class clear: Example: class PrintAttributeMeta : def __new__ ( cls , class_name , bases , attributes ): print ( "Attributes received in PrintAttributeMeta:" , attributes ) return type ( class_name , bases , attributes ) class Dummy ( metaclass = PrintAttributeMeta ): class_var1 = "lorem" class_var2 = 45 Output: Attributes received in PrintAttributeMeta: {'__module__': '__main__', '__qualname__': 'Dummy', 'class_var1': 'lorem', 'class_var2': 45} This meta class simply prints all attributes, and then creates and returns a new class by using the type() function. Note that there’s output without creating an instance of the class because the __new__() method is called when the class is created. Using Metaclasses, the user can perform multitude of validations and modification such as changing variable names, raising error on receiving an invalid data type, checking inheritance levels, etc. For this example, we will write a Metaclass that raises a ValueError when a negative value is received in attributes . class PositiveNumbersMeta : def __new__ ( cls , class_name , bases , attributes ): for attr , value in attributes . items (): if isinstance ( value , ( int , float )) and value < 0 : raise ValueError ( f "Negative values are not allowed in { class_name } " ) return type ( class_name , bases , attributes ) class MyClass ( metaclass = PositiveNumbersMeta ): class_var1 = 23 class_var2 = - 1 Output: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in __new__ ValueError: Negative values are not allowed in MyClass We could also change attributes: class PositiveNumbersMeta : def __new__ ( cls , class_name , bases , attributes ): for attr , value in attributes . items (): if isinstance ( value , ( int , float )) and value < 0 : attributes [ attr ] = - value return type ( class_name , bases , attributes ) class MyClass ( metaclass = PositiveNumbersMeta ): class_var1 = 23 class_var2 = - 1 a = MyClass () print ( a . class_var2 ) # 1 5. Use proper OOP principles ¶ In the previous example we are calling type directly and aren't overriding or calling the parent's __new__ . Let's do that instead: class PositiveNumbersMeta ( type ): def __new__ ( cls , class_name , bases , attributes ): for attr , value in attributes . items (): if isinstance ( value , ( int , float )) and value < 0 : attributes [ attr ] = - value return super . __new__ ( class_name , bases , attributes ) Now our metaclass inherits from type , and by calling the super.__new__() we are using the OOP syntax and call the __new__() function of the base class. There are several reasons to do so: The intention is clear. When you read PositiveNumbersMeta, you know what's going to follow You can use OOP. Metaclass can inherit from metaclass, override parent methods, etc. Metaclasses can even use other metaclasses. You can structure your code better. You never use metaclasses for something as trivial as the above example. It's usually for something complicated. Having the ability to make several methods and group them in one class is very useful to make the code easier to read. You can hook on __new__ , __init__ and __call__ . Which will allow you to do different stuff, Even if usually you can do it all in __new__ , some people are just more comfortable using __init__ . Resource: Here's a great StackOverflow answer that explains it well. Note that Metaclasses are usually only used in more complex scenarios, and in 99% of cases you don't have to worry about it. One common use case is when creating an API. A typical example of this is the Django ORM . FREE VS Code / PyCharm Extensions I Use ✅ Write cleaner code with Sourcery, instant refactoring suggestions: Link* Python Problem-Solving Bootcamp 🚀 Solve 42 programming puzzles over the course of 21 days: Link* * These are affiliate link. By clicking on it you will not have any additional costs. Instead, you will support my project. Thank you! 🙏
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Type and OOP](https://www.python-engineer.com/posts/metaclasses-python/#1-type-and-oop) - [2\. Understanding the type() function](https://www.python-engineer.com/posts/metaclasses-python/#2-understanding-the-type-function) - [Creating classes dynamically using type()](https://www.python-engineer.com/posts/metaclasses-python/#creating-classes-dynamically-using-type) - [3\. Creating Metaclass](https://www.python-engineer.com/posts/metaclasses-python/#3-creating-metaclass) - [4\. Modifying a class state using Metaclass](https://www.python-engineer.com/posts/metaclasses-python/#4-modifying-a-class-state-using-metaclass) - [5\. Use proper OOP principles](https://www.python-engineer.com/posts/metaclasses-python/#5-use-proper-oop-principles) - [6\. 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[Text Classification - NLP Tutorial - TensorFlow Beginner 11](https://www.python-engineer.com/courses/tensorflowbeginner/11-nlp-text-classification/) - Tags Tags - [Tags](https://www.python-engineer.com/tags/) - Authors Authors - [Authors](https://www.python-engineer.com/authors/) - [About](https://www.python-engineer.com/about/) - [Newsletter](https://www.python-engineer.com/newsletter/) On this page - [1\. Type and OOP](https://www.python-engineer.com/posts/metaclasses-python/#1-type-and-oop) - [2\. Understanding the type() function](https://www.python-engineer.com/posts/metaclasses-python/#2-understanding-the-type-function) - [Creating classes dynamically using type()](https://www.python-engineer.com/posts/metaclasses-python/#creating-classes-dynamically-using-type) - [3\. Creating Metaclass](https://www.python-engineer.com/posts/metaclasses-python/#3-creating-metaclass) - [4\. Modifying a class state using Metaclass](https://www.python-engineer.com/posts/metaclasses-python/#4-modifying-a-class-state-using-metaclass) - [5\. Use proper OOP principles](https://www.python-engineer.com/posts/metaclasses-python/#5-use-proper-oop-principles) - [6\. When to use metaclasses?](https://www.python-engineer.com/posts/metaclasses-python/#6-when-to-use-metaclasses) # What are metaclasses in Python A beginners guide to metaclasses in Python. ![Pratik Choudhari](https://www.python-engineer.com/img/pratik.webp) Pratik Choudhari · · · February 07, 2022 · 9 min read [Python](https://www.python-engineer.com/tags/#python) [Basics](https://www.python-engineer.com/tags/#basics) * Metaclass is an advanced Python concept and is not used in day-to-day programming tasks. They allow a user to precisely define a class state, change its attributes and control method behavior. \#more In this article, we will understand how metaclasses can be created and used in Python. ## 1\. Type and OOP[¶](https://www.python-engineer.com/posts/metaclasses-python/#1-type-and-oop "Permanent link") Everything is an object in Python, including classes. Hence, if classes are an object, they must be created by another class also called as Metaclass. ![class hierarchy](https://www.python-engineer.com/img/2022-01-29-metaclasses/metaclass.webp) So, a metaclass is just another class that creates class objects. Usually, `type` is the built-in metaclass Python uses for every class. But we can also create our own metaclasses with custom behavior. ## 2\. Understanding the `type()` function[¶](https://www.python-engineer.com/posts/metaclasses-python/#2-understanding-the-type-function "Permanent link") To fully understand Metaclasses, one needs to thoroughly understand `type()`. In Python, `type()` is used to identify the data type of a variable. If we have the following code ``` ``` Console output would be: ``` <class 'int'> ``` Indicating `var` is an instance of the class `int`. In the case of a user-defined class, we get the following output. ``` ``` ``` ``` Here, we can see `foo_obj` is correctly identified as an instance of class `Foo`, but the type of class `Foo` indicates that it is an instance of class `type`, establishing the point that classes are indeed created by other classes. ## Creating classes dynamically using `type()`[¶](https://www.python-engineer.com/posts/metaclasses-python/#creating-classes-dynamically-using-type "Permanent link") This is one more concept we should know before diving into metaclasses. This usage of `type()` is fairly unknown. The `type()` function can take three arguments and can be used to create classes dynamically: ``` type(name: str, bases: tuple, attributes: dict) ``` - `name` indicates the name of a class - `bases` is a tuple of classes to be inherited - `attributes` is a dictionary including all variables and methods to be declared inside the new class Example: ``` ``` Output: ``` lorem ipsum sit domat ``` ## 3\. Creating Metaclass[¶](https://www.python-engineer.com/posts/metaclasses-python/#3-creating-metaclass "Permanent link") The main purpose of a metaclass is to change the class automatically when it's created. To use class as a metaclass we pass `metaclass=ClassName` as a class parameter. The metaclass itself is implemented like a normal class with any behavior we want. Usually the `__new__` function is implemented. The name of the metaclass should ideally make the intent of the class clear: Example: ``` ``` Output: ``` Attributes received in PrintAttributeMeta: {'__module__': '__main__', '__qualname__': 'Dummy', 'class_var1': 'lorem', 'class_var2': 45} ``` This meta class simply prints all attributes, and then creates and returns a new class by using the `type()` function. Note that there’s output without creating an instance of the class because the `__new__()` method is called when the class is created. ## 4\. Modifying a class state using Metaclass[¶](https://www.python-engineer.com/posts/metaclasses-python/#4-modifying-a-class-state-using-metaclass "Permanent link") Using Metaclasses, the user can perform multitude of validations and modification such as changing variable names, raising error on receiving an invalid data type, checking inheritance levels, etc. For this example, we will write a Metaclass that raises a `ValueError` when a negative value is received in `attributes`. ``` ``` Output: ``` ``` We could also change attributes: ``` ``` ## 5\. Use proper OOP principles[¶](https://www.python-engineer.com/posts/metaclasses-python/#5-use-proper-oop-principles "Permanent link") In the previous example we are calling `type` directly and aren't overriding or calling the parent's `__new__`. Let's do that instead: ``` ``` Now our metaclass inherits from `type`, and by calling the `super.__new__()` we are using the OOP syntax and call the `__new__()` function of the base class. ## 6\. When to use metaclasses?[¶](https://www.python-engineer.com/posts/metaclasses-python/#6-when-to-use-metaclasses "Permanent link") There are several reasons to do so: - The intention is clear. When you read PositiveNumbersMeta, you know what's going to follow - You can use OOP. Metaclass can inherit from metaclass, override parent methods, etc. Metaclasses can even use other metaclasses. - You can structure your code better. You never use metaclasses for something as trivial as the above example. It's usually for something complicated. Having the ability to make several methods and group them in one class is very useful to make the code easier to read. - You can hook on `__new__`, `__init__` and `__call__`. Which will allow you to do different stuff, Even if usually you can do it all in `__new__`, some people are just more comfortable using `__init__`. Resource: Here's a great [StackOverflow answer](https://stackoverflow.com/questions/100003/what-are-metaclasses-in-python) that explains it well. Note that Metaclasses are usually only used in more complex scenarios, and in 99% of cases you don't have to worry about it. One common use case is when creating an API. A typical example of this is the [Django ORM](https://docs.djangoproject.com/en/4.0/topics/db/models/). * FREE VS Code / PyCharm Extensions I Use ✅ Write cleaner code with Sourcery, instant refactoring suggestions: [Link\](https://sourcery.ai/?utm_source=youtube&utm_campaign=pythonengineer) ** Python Problem-Solving Bootcamp 🚀 Solve 42 programming puzzles over the course of 21 days: [Link\](https://www.python-engineer.com/go/sponsor-pythonbootcamp) \ These are affiliate link. By clicking on it you will not have any additional costs. Instead, you will support my project. 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Readable Markdown	A beginners guide to metaclasses in Python. [Python](https://www.python-engineer.com/tags/#python) [Basics](https://www.python-engineer.com/tags/#basics) * Metaclass is an advanced Python concept and is not used in day-to-day programming tasks. They allow a user to precisely define a class state, change its attributes and control method behavior. In this article, we will understand how metaclasses can be created and used in Python. ## 1\. Type and OOP[¶](https://www.python-engineer.com/posts/metaclasses-python/#1-type-and-oop "Permanent link") Everything is an object in Python, including classes. Hence, if classes are an object, they must be created by another class also called as Metaclass. ![class hierarchy](https://www.python-engineer.com/img/2022-01-29-metaclasses/metaclass.webp) So, a metaclass is just another class that creates class objects. Usually, `type` is the built-in metaclass Python uses for every class. But we can also create our own metaclasses with custom behavior. ## 2\. Understanding the `type()` function[¶](https://www.python-engineer.com/posts/metaclasses-python/#2-understanding-the-type-function "Permanent link") To fully understand Metaclasses, one needs to thoroughly understand `type()`. In Python, `type()` is used to identify the data type of a variable. If we have the following code ``` ``` Console output would be: ``` <class 'int'> ``` Indicating `var` is an instance of the class `int`. In the case of a user-defined class, we get the following output. ``` ``` ``` ``` Here, we can see `foo_obj` is correctly identified as an instance of class `Foo`, but the type of class `Foo` indicates that it is an instance of class `type`, establishing the point that classes are indeed created by other classes. ## Creating classes dynamically using `type()`[¶](https://www.python-engineer.com/posts/metaclasses-python/#creating-classes-dynamically-using-type "Permanent link") This is one more concept we should know before diving into metaclasses. This usage of `type()` is fairly unknown. The `type()` function can take three arguments and can be used to create classes dynamically: ``` type(name: str, bases: tuple, attributes: dict) ``` - `name` indicates the name of a class - `bases` is a tuple of classes to be inherited - `attributes` is a dictionary including all variables and methods to be declared inside the new class Example: ``` ``` Output: ``` lorem ipsum sit domat ``` The main purpose of a metaclass is to change the class automatically when it's created. To use class as a metaclass we pass `metaclass=ClassName` as a class parameter. The metaclass itself is implemented like a normal class with any behavior we want. Usually the `__new__` function is implemented. The name of the metaclass should ideally make the intent of the class clear: Example: ``` ``` Output: ``` Attributes received in PrintAttributeMeta: {'__module__': '__main__', '__qualname__': 'Dummy', 'class_var1': 'lorem', 'class_var2': 45} ``` This meta class simply prints all attributes, and then creates and returns a new class by using the `type()` function. Note that there’s output without creating an instance of the class because the `__new__()` method is called when the class is created. Using Metaclasses, the user can perform multitude of validations and modification such as changing variable names, raising error on receiving an invalid data type, checking inheritance levels, etc. For this example, we will write a Metaclass that raises a `ValueError` when a negative value is received in `attributes`. ``` ``` Output: ``` ``` We could also change attributes: ``` ``` ## 5\. Use proper OOP principles[¶](https://www.python-engineer.com/posts/metaclasses-python/#5-use-proper-oop-principles "Permanent link") In the previous example we are calling `type` directly and aren't overriding or calling the parent's `__new__`. Let's do that instead: ``` ``` Now our metaclass inherits from `type`, and by calling the `super.__new__()` we are using the OOP syntax and call the `__new__()` function of the base class. There are several reasons to do so: - The intention is clear. When you read PositiveNumbersMeta, you know what's going to follow - You can use OOP. Metaclass can inherit from metaclass, override parent methods, etc. Metaclasses can even use other metaclasses. - You can structure your code better. You never use metaclasses for something as trivial as the above example. It's usually for something complicated. Having the ability to make several methods and group them in one class is very useful to make the code easier to read. - You can hook on `__new__`, `__init__` and `__call__`. Which will allow you to do different stuff, Even if usually you can do it all in `__new__`, some people are just more comfortable using `__init__`. Resource: Here's a great [StackOverflow answer](https://stackoverflow.com/questions/100003/what-are-metaclasses-in-python) that explains it well. Note that Metaclasses are usually only used in more complex scenarios, and in 99% of cases you don't have to worry about it. One common use case is when creating an API. A typical example of this is the [Django ORM](https://docs.djangoproject.com/en/4.0/topics/db/models/). * FREE VS Code / PyCharm Extensions I Use ✅ Write cleaner code with Sourcery, instant refactoring suggestions: [Link\](https://sourcery.ai/?utm_source=youtube&utm_campaign=pythonengineer) ** Python Problem-Solving Bootcamp 🚀 Solve 42 programming puzzles over the course of 21 days: [Link\](https://www.python-engineer.com/go/sponsor-pythonbootcamp) \ These are affiliate link. By clicking on it you will not have any additional costs. Instead, you will support my project. Thank you! 🙏 ***
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