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Introduction to MetaClasses Sanyam Khurana ErSanyamKhurana CuriousLearner PyGotham 2020 | New York City | PyGotham TV

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def __who_am_i__(): • One of you! A part of the community • Goes by “CuriousLearner” over the web / IRC • FOSS contributor • Bug triager for CPython • Contributor to Django and Mozilla’s ecosystem of projects • Maintainer for django-phone-verify

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MetaProgramming ErSanyamKhurana CuriousLearner

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MetaClasses ErSanyamKhurana CuriousLearner

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Everything is an “Object” ErSanyamKhurana CuriousLearner

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Everything is an “Object” ErSanyamKhurana CuriousLearner int str
 float functions
 class

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Classes are objects ErSanyamKhurana CuriousLearner

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Classes are objects ErSanyamKhurana CuriousLearner

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Classes are objects ErSanyamKhurana CuriousLearner

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Classes are objects ErSanyamKhurana CuriousLearner Assign class to a variable Copy class Add attributes to a class Pass class as parameters to a function Classes are objects

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ErSanyamKhurana CuriousLearner Django Phone Verify

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The esoteric `type` ErSanyamKhurana CuriousLearner • You might already have seen or used type in your Python code

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ErSanyamKhurana CuriousLearner Classes are objects

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What’s the type of built-in classes ErSanyamKhurana CuriousLearner

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type Name of the class to be created

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type Name of the class to be created Tuple of all the base classes that the new class should inherit from

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type Name of the class to be created Tuple of all the base classes that the new class should inherit from Dictionary of all the attributes of new class in key value pairs

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type Both are equivalent

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How to define metaclass for your class? ErSanyamKhurana CuriousLearner

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How to define metaclass for your class? ErSanyamKhurana CuriousLearner

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How to define custom meta class for your class? ErSanyamKhurana CuriousLearner

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The esoteric `type` ErSanyamKhurana CuriousLearner Creating classes using type That’s the entire magic of dynamic class creation that happens when the interpreter sees the block of code indicating a class. type is the default metaclass

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The world of Analogies ErSanyamKhurana CuriousLearner child : parent :: parent : grandparent object : class :: class : metaclass

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What’s the type of type ErSanyamKhurana CuriousLearner

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What’s the type of type ErSanyamKhurana CuriousLearner

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What’s the type of type ErSanyamKhurana CuriousLearner

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What’s the type of type ErSanyamKhurana CuriousLearner

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What’s the type of type ErSanyamKhurana CuriousLearner Inception

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What’s the type of type ErSanyamKhurana CuriousLearner

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What’s a MetaClass : Revisited ErSanyamKhurana CuriousLearner It’s a class factory. Provides a way to create classes on the fly. Let’s you hook-into the class creation logic to validate things ensuring the newly derived class conforms to a certain API standard.

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Singleton Classes ErSanyamKhurana CuriousLearner

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Singleton Classes ErSanyamKhurana CuriousLearner

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Singleton Classes ErSanyamKhurana CuriousLearner

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Singleton Classes ErSanyamKhurana CuriousLearner

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Singleton Classes ErSanyamKhurana CuriousLearner

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Singleton Classes ErSanyamKhurana CuriousLearner

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Singleton Classes ErSanyamKhurana CuriousLearner

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Abstract Base Classes ErSanyamKhurana CuriousLearner

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Abstract Base Classes ErSanyamKhurana CuriousLearner

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Deeper look at MetaClasses • Called before class body is executed • Must return an instance of dict with local namespace for all the code in the class body. • PEP-3115 — Metaclasses in Python 3000 ErSanyamKhurana CuriousLearner __prepare__ method

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Deeper look at MetaClasses ErSanyamKhurana CuriousLearner

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Deeper look at MetaClasses • The __new__ method is the constructor — generates the new instance • __init__ is just a initializer — called after instance is created • __new__ is looked up statically on the instance (class) itself and not up in the hierarchy such as in the metaclass. ErSanyamKhurana CuriousLearner __new__ & __init__ method

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ErSanyamKhurana CuriousLearner Deeper look at Instance Creation Image Source: https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/

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ErSanyamKhurana CuriousLearner Deeper look at Class Creation Image Source: https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/

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ErSanyamKhurana CuriousLearner MetaClass: A deeper look Source: https://github.com/python/cpython/blob/3.9/Lib/enum.py#L122

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ErSanyamKhurana CuriousLearner MetaClass: A deeper look Source: https://github.com/python/cpython/blob/3.9/Lib/enum.py#L122

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ErSanyamKhurana CuriousLearner MetaClass: A deeper look Source: https://github.com/python/cpython/blob/3.9/Lib/enum.py#L122

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ErSanyamKhurana CuriousLearner MetaClass: A deeper look Source: https://github.com/python/cpython/blob/3.9/Lib/enum.py#L122 __iter__

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ErSanyamKhurana CuriousLearner MetaClass: A deeper look Source: https://github.com/python/cpython/blob/3.9/Lib/enum.py#L122 __iter__

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ErSanyamKhurana CuriousLearner MetaClass: A deeper look Source: https://github.com/python/cpython/blob/3.9/Lib/enum.py#L122 __iter__ __len__

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Multiple Inheritance in MetaClasses ErSanyamKhurana CuriousLearner

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Multiple Inheritance in Metaclasses ErSanyamKhurana CuriousLearner

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Multiple Inheritance in Metaclasses Only works when you have single root MetaClass from which all base classes inherit from. ErSanyamKhurana CuriousLearner

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Multiple Inheritance in Metaclasses Only works when you have single root MetaClass from which all base classes inherit from. ErSanyamKhurana CuriousLearner

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When to use MetaClasses? Before you uses metaclass, ask yourself: - Can I accomplish what I want with a decorator? If yes, then choose decorator. - Am I trying to provide an API for programmers to use (using my library/framework), specifically for derived classes? Would MetaClass reduce the amount of code an application programmer has to write? If yes, then use metaclasses. ErSanyamKhurana CuriousLearner

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Use cases of MetaClasses • Validation framework for attributes of classes • Hook into the class creation process to control either the inheritance to or fro from the custom class. • Emulating behaviour in other programming languages, such as making a final class as seen in Java. • Hook into the class creation process to maintain order of class definitions (not instance creations) • Controlling meta stuff about the class like enforcing all the private methods of the class to have docstrings. ErSanyamKhurana CuriousLearner

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References • https://stackoverflow.com/questions/100003/what-are-metaclasses-in- python • https://stackoverflow.com/questions/392160/what-are-some-concrete- use-cases-for-metaclasses • https://eli.thegreenplace.net/2011/08/14/python-metaclasses-by- example/ • https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/ • https://www.python.org/dev/peps/pep-3115/ • https://docs.python.org/3/reference/datamodel.html#customizing-class- creation ErSanyamKhurana CuriousLearner

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Subscribe for updates: SanyamKhurana.com/python.html ErSanyamKhurana CuriousLearner

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Many Thanks! Slides available at: https://speakerdeck.com/CuriousLearner/ introduction-to-metaclasses Reach out to me at: [email protected] ErSanyamKhurana CuriousLearner