{"id":2964,"date":"2021-11-17T01:38:21","date_gmt":"2021-11-17T01:38:21","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2964"},"modified":"2025-03-28T03:36:41","modified_gmt":"2025-03-28T03:36:41","slug":"python-metaclass","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-metaclass\/","title":{"rendered":"Python Metaclass"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python metaclass and understand how Python uses the metaclasses to create other classes.<\/p>\n\n\n\n

Introduction to the Python Metaclass #<\/a><\/h2>\n\n\n\n

A metaclass is a class<\/a> that creates other classes. By default, Python uses the type<\/a><\/code> metaclass to create other classes.<\/p>\n\n\n\n

For example, the following defines a Person<\/code> class:<\/p>\n\n\n

class<\/span> Person<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
When Python executes the code, it uses the type<\/code> metaclass to create the Person<\/code> class. The reason is that the Person<\/code> class uses the type<\/code> metaclass by default. <\/p>\n\n\n\n
The explicit Person<\/code> class definition looks like this:<\/p>\n\n\n
class<\/span> Person<\/span>(object, metaclass=type)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The metaclass<\/code> argument allows you to specify which metaclass class to use to define the class. Therefore, you can create a custom metaclass that has its own logic to create other classes. By using a custom metaclass, you can inject functionality into the class creation process.<\/p>\n\n\n\n
Python metaclass example #<\/a><\/h2>\n\n\n\n
First, define a custom metaclass called Human<\/code> that has the freedom<\/code> attribute sets to True<\/code> by default:<\/p>\n\n\n
class<\/span> Human<\/span>(type)<\/span>:<\/span>\n    def<\/span> __new__<\/span>(mcs, name, bases, class_dict)<\/span>:<\/span>\n        class_ = super().__new__(mcs, name, bases, class_dict)\n        class_.freedom = True<\/span>\n        return<\/span> class_<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, define the Person<\/code> class that uses the Human<\/code> metaclass:<\/p>\n\n\n
class<\/span> Person<\/span>(object, metaclass=Human)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
pprint(Person.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
mappingproxy({'__dict__'<\/span>: <attribute '__dict__'<\/span> of 'Person'<\/span> objects>,\n              '__doc__'<\/span>: None<\/span>,\n              '__init__'<\/span>: <function Person.__init__ at 0x000001E716C71670<\/span>>,\n              '__module__'<\/span>: '__main__'<\/span>,\n              '__weakref__'<\/span>: <attribute '__weakref__'<\/span> of 'Person'<\/span> objects>,\n              'freedom'<\/span>: True<\/span>})<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Put it all together.<\/p>\n\n\n
from<\/span> pprint import<\/span> pprint\n\n\nclass<\/span> Human<\/span>(type)<\/span>:<\/span>\n    def<\/span> __new__<\/span>(mcs, name, bases, class_dict)<\/span>:<\/span>\n        class_ = super().__new__(mcs, name, bases, class_dict)\n        class_.freedom = True<\/span>\n        return<\/span> class_\n\n\nclass<\/span> Person<\/span>(object, metaclass=Human)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age\n\n\npprint(Person.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
mappingproxy({'__dict__'<\/span>: <attribute '__dict__'<\/span> of 'Person'<\/span> objects>,\n              '__doc__'<\/span>: None,\n              '__init__'<\/span>: <function<\/span> Person<\/span>.__init__<\/span> at<\/span> 0x10e2db0<\/span>>,\n              '__module__<\/span>': '__main__<\/span>',\n              '__weakref__<\/span>': <attribute<\/span> '__weakref__<\/span>' of<\/span> 'Person<\/span>' objects<\/span>>,\n              'freedom<\/span>': True<\/span>})<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Metaclass Parameters #<\/a><\/h2>\n\n\n\n
To pass parameters to a metaclass, you use the keyword arguments. For example, the following redefines the Human<\/code> metaclass that accepts keyword arguments, where each argument becomes a class variable:<\/p>\n\n\n
class<\/span> Human<\/span>(type)<\/span>:<\/span>\n    def<\/span> __new__<\/span>(mcs, name, bases, class_dict, **kwargs)<\/span>:<\/span>\n        class_ = super().__new__(mcs, name, bases, class_dict)\n        if<\/span> kwargs:\n            for<\/span> name, value in<\/span> kwargs.items():\n                setattr(class_, name, value)\n        return<\/span> class_<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The following uses the Human<\/code> metaclass to create a Person<\/code> class with the country<\/code> and freedom<\/code> class variables set to USA<\/code> and True<\/code> respectively:<\/p>\n\n\n
class<\/span> Person<\/span>(object, metaclass=Human, country='USA'<\/span>, freedom=True)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Here are Person<\/code> class variables:<\/p>\n\n\n
pprint(Person.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
mappingproxy({'__dict__'<\/span>: <attribute '__dict__'<\/span> of 'Person'<\/span> objects>,\n              '__doc__'<\/span>: None<\/span>,\n              '__init__'<\/span>: <function Person.__init__ at 0x0000018A334235E0<\/span>>,\n              '__module__'<\/span>: '__main__'<\/span>,\n              '__weakref__'<\/span>: <attribute '__weakref__'<\/span> of 'Person'<\/span> objects>,\n              'country'<\/span>: 'USA'<\/span>,\n              'freedom'<\/span>: True<\/span>})<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Put it all together.<\/p>\n\n\n
from<\/span> pprint import<\/span> pprint\n\n\nclass<\/span> Human<\/span>(type)<\/span>:<\/span>\n    def<\/span> __new__<\/span>(mcs, name, bases, class_dict, **kwargs)<\/span>:<\/span>\n        class_ = super().__new__(mcs, name, bases, class_dict)\n        if<\/span> kwargs:\n            for<\/span> name, value in<\/span> kwargs.items():\n                setattr(class_, name, value)\n        return<\/span> class_\n\n\nclass<\/span> Person<\/span>(object, metaclass=Human, freedom=True, country='USA'<\/span>)<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age\n\n\npprint(Person.__dict__)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
When to use metaclasses #<\/a><\/h2>\n\n\n\n
Here’s the quote from Tim Peter<\/a> who wrote the Zen of Python:<\/p>\n\n\n\n
\n
Metaclasses are deeper magic that 99% of users should never worry about it. If you wonder whether you need them, you don’t (the people who actually need them to know with certainty that they need them and don’t need an explanation about why).<\/p>\nTim Peter<\/cite><\/blockquote>\n\n\n\n
In practice, you often don’t need to use metaclasses unless you maintain or develop the core of large frameworks such as Django.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • A metaclass is a class that creates other classes.<\/li>\n<\/ul>\n
    \n\t
    \n\t\t
    Was this tutorial helpful ?<\/div>\n\t\t
    \n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t Yes <\/span>\n\t\t\t<\/button>\n\n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t No <\/span>\n\t\t\t<\/button>\n\t\t<\/div>\n\t<\/header>\n\n\t
    \n\t\t
    \n\t\t\t
    <\/div>\n\t\t\t
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
The Person<\/code>class will have the freedom<\/code> attribute as shown in the class variables<\/a>:<\/p>\n\n\n
Note that the __new__<\/a><\/code> method returns a new class or a class object.<\/p>\n\n\n\n