Class as decorator in python

Decorators are a very powerful and useful tool in Python since it allows programmers to modify the behaviour of function or class. Decorators allow us to wrap another function in order to extend the behaviour of the wrapped function, without permanently modifying it. 
We can define a decorator as a class in order to do that, we have to use a __call__ method of classes. When a user needs to create an object that acts as a function then function decorator needs to return an object that acts like a function, so __call__ can be useful. For Example 

Python3

# Python program showing
# use of __call__() method

class MyDecorator:
    def __init__(self, function):
        self.function = function

    def __call__(self):

        # We can add some code
        # before function call

        self.function()

        # We can also add some code
        # after function call.


# adding class decorator to the function
@MyDecorator
def function():
    print("GeeksforGeeks")

function()

Output:

GeeksforGeeks

Class Decorator with *args and **kwargs :
In order to use class decorator with argument *args and **kwargs we used a __call__ function and passed both the argument in a given function 

Python3

# Python program showing
# class decorator with *args
# and **kwargs

class MyDecorator:
    def __init__(self, function):
        self.function = function

    def __call__(self, *args, **kwargs):

        # We can add some code
        # before function call

        self.function(*args, **kwargs)

        # We can also add some code
        # after function call.


# adding class decorator to the function
@MyDecorator
def function(name, message ='Hello'):
    print("{}, {}".format(message, name))

function("geeks_for_geeks", "hello")

Output:

hello, geeks_for_geeks

Class Decorator with return statement :
In the given example the functions did not return anything so there is not any issue, but one may need the returned value. So we use return statement with the class decorator. 

Python3

# Python program showing
# class decorator with
# return statement

class SquareDecorator:

    def __init__(self, function):
        self.function = function

    def __call__(self, *args, **kwargs):

        # before function
        result = self.function(*args, **kwargs)

        # after function
        return result

 # adding class decorator to the function
@SquareDecorator
def get_square(n):
    print("given number is:", n)
    return n * n

print("Square of number is:", get_square(195))

Output:

given number is: 195
Square of number is: 38025

Using class Decorators to print Time required to execute a program :
In order to print time required to execute a program, we use __call__ function and use a time module so that we can get a execute time of a program 

Python3

# Python program to execute
# time of a program

# importing time module
from time import time
class Timer:

    def __init__(self, func):
        self.function = func

    def __call__(self, *args, **kwargs):
        start_time = time()
        result = self.function(*args, **kwargs)
        end_time = time()
        print("Execution took {} seconds".format(end_time-start_time))
        return result


# adding a decorator to the function
@Timer
def some_function(delay):
    from time import sleep

    # Introducing some time delay to
    # simulate a time taking function.
    sleep(delay)

some_function(3)

Output:

Execution took 3.003122091293335 seconds

Checking error parameter using class decorator :
This type of class decorator is most frequently used. This decorator checks parameters before executing the function preventing the function to become overloaded and enables it to store only logical and necessary statements.

Python3

# Python program checking
# error parameter using
# class decorator

class ErrorCheck:

    def __init__(self, function):
        self.function = function

    def __call__(self, *params):
        if any([isinstance(i, str) for i in params]):
            raise TypeError("parameter cannot be a string !!")
        else:
            return self.function(*params)


@ErrorCheck
def add_numbers(*numbers):
    return sum(numbers)

#  returns 6
print(add_numbers(1, 2, 3))

# raises Error.
print(add_numbers(1, '2', 3))

Output :

6
TypeError: parameter cannot be a string !!