{"id":1017,"date":"2020-11-10T03:36:11","date_gmt":"2020-11-10T03:36:11","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=1017"},"modified":"2025-03-27T13:56:03","modified_gmt":"2025-03-27T13:56:03","slug":"python-class-decorators","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-class-decorators\/","title":{"rendered":"Python Class Decorators"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about Python class decorators. After the tutorial, you’ll know how to define classes as decorators.<\/p>\n\n\n\n

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

So far you have learned how to use functions<\/a> to define decorators<\/a>. <\/p>\n\n\n\n

For example, the following star<\/code> function prints out a number of *<\/code> characters before and after calling the decorated function:<\/p>\n\n\n

def<\/span> star<\/span>(n)<\/span>:<\/span>\n    def<\/span> decorate<\/span>(fn)<\/span>:<\/span>\n        def<\/span> wrapper<\/span>(*args, **kwargs)<\/span>:<\/span>\n            print(n*'*'<\/span>)\n            result = fn(*args, **kwargs)\n            print(result)\n            print(n*'*'<\/span>)\n            return<\/span> result\n        return<\/span> wrapper\n    return<\/span> decorate<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The following illustrates how to use the star<\/code> decorator factory:<\/p>\n\n\n
def<\/span> star<\/span>(n)<\/span>:<\/span>\n    def<\/span> decorate<\/span>(fn)<\/span>:<\/span>\n        def<\/span> wrapper<\/span>(*args, **kwargs)<\/span>:<\/span>\n            print(n*'*'<\/span>)\n            result = fn(*args, **kwargs)\n            print(result)\n            print(n*'*'<\/span>)\n            return<\/span> result\n        return<\/span> wrapper\n    return<\/span> decorate\n\n@star(5)<\/span>\ndef<\/span> add<\/span>(a, b)<\/span>:<\/span>\n    return<\/span> a + b\n\n\nadd(10<\/span>, 20<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
*****\n30\n*****<\/code><\/span><\/pre>\n\n\n
The star()<\/code> decorator factory takes an argument and returns a callable. The callable takes an argument (fn<\/code>) which is a function that will be decorated. Also, the callable can access the argument (n<\/code>) passed to the decorator factory.<\/p>\n\n\n\n
A class instance can be a callable when it implements the __call__<\/a><\/code> method. Therefore, you can make the __call__<\/code> method as a decorator.<\/p>\n\n\n\n
The following example rewrites the star<\/code> decorator factory using a class instead:<\/p>\n\n\n
class<\/span> Star<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, n)<\/span>:<\/span>\n        self.n = n\n\n    def<\/span> __call__<\/span>(self, fn)<\/span>:<\/span>\n        def<\/span> wrapper<\/span>(*args, **kwargs)<\/span>:<\/span>\n            print(self.n*'*'<\/span>)\n            result = fn(*args, **kwargs)\n            print(result)\n            print(self.n*'*'<\/span>)\n            return<\/span> result\n        return<\/span> wrapper<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
And you can use the Star<\/code> class as a decorator like this:<\/p>\n\n\n
@Star(5)<\/span>\ndef<\/span> add<\/span>(a, b)<\/span>:<\/span>\n    return<\/span> a + b<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The @Star(5)<\/code> returns an instance of the Star<\/code> class. That instance is a callable, so you can do something like:<\/p>\n\n\n
add = Star(5<\/span>)(add)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
So you can use callable classes to decorate functions.<\/p>\n\n\n\n
Put it all together:<\/p>\n\n\n
from<\/span> functools import<\/span> wraps\n\n\nclass<\/span> Star<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, n)<\/span>:<\/span>\n        self.n = n\n\n    def<\/span> __call__<\/span>(self, fn)<\/span>:<\/span>\n        @wraps(fn)<\/span>\n        def<\/span> wrapper<\/span>(*args, **kwargs)<\/span>:<\/span>\n            print(self.n*'*'<\/span>)\n            result = fn(*args, **kwargs)\n            print(result)\n            print(self.n*'*'<\/span>)\n            return<\/span> result\n        return<\/span> wrapper\n\n\n@Star(5)<\/span>\ndef<\/span> add<\/span>(a, b)<\/span>:<\/span>\n    return<\/span> a + b\n\n\nadd(10<\/span>, 20<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
*****\n30\n*****<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
The star<\/code> is a decorator factory<\/a> that returns a decorator. It accepts an argument that specifies the number of *<\/code> characters to display.<\/p>\n\n\n\n