Python Class Decorators

Summary: in this tutorial, you’ll learn about Python class decorators. After the tutorial, you’ll know how to define classes as decorators.

Introduction to the Python class decorators #

So far you have learned how to use functions to define decorators.

For example, the following star function prints out a number of * characters before and after calling the decorated function:

def star(n):
    def decorate(fn):
        def wrapper(*args, **kwargs):
            print(n*'*')
            result = fn(*args, **kwargs)
            print(result)
            print(n*'*')
            return result
        return wrapper
    return decorateCode language: Python (python)

The star is a decorator factory that returns a decorator. It accepts an argument that specifies the number of * characters to display.

The following illustrates how to use the star decorator factory:

def star(n):
    def decorate(fn):
        def wrapper(*args, **kwargs):
            print(n*'*')
            result = fn(*args, **kwargs)
            print(result)
            print(n*'*')
            return result
        return wrapper
    return decorate

@star(5)
def add(a, b):
    return a + b


add(10, 20)Code language: Python (python)

Try it

Output:

*****
30
*****

The star() decorator factory takes an argument and returns a callable. The callable takes an argument (fn) which is a function that will be decorated. Also, the callable can access the argument (n) passed to the decorator factory.

A class instance can be a callable when it implements the __call__ method. Therefore, you can make the __call__ method as a decorator.

The following example rewrites the star decorator factory using a class instead:

class Star:
    def __init__(self, n):
        self.n = n

    def __call__(self, fn):
        def wrapper(*args, **kwargs):
            print(self.n*'*')
            result = fn(*args, **kwargs)
            print(result)
            print(self.n*'*')
            return result
        return wrapperCode language: Python (python)

And you can use the Star class as a decorator like this:

@Star(5)
def add(a, b):
    return a + bCode language: Python (python)

The @Star(5) returns an instance of the Star class. That instance is a callable, so you can do something like:

add = Star(5)(add)Code language: Python (python)

So you can use callable classes to decorate functions.

Put it all together:

from functools import wraps


class Star:
    def __init__(self, n):
        self.n = n

    def __call__(self, fn):
        @wraps(fn)
        def wrapper(*args, **kwargs):
            print(self.n*'*')
            result = fn(*args, **kwargs)
            print(result)
            print(self.n*'*')
            return result
        return wrapper


@Star(5)
def add(a, b):
    return a + b


add(10, 20)Code language: Python (python)

Try it

Output:

*****
30
*****Code language: plaintext (plaintext)

Summary #

• Use callable classes as decorators by implementing the __call__ method.
• Pass the decorator arguments to the __init__ method.