class as decorator

Decorator

Decorator is a callable which can modify the function, method, class on runtime. Most ofthe decorators uses closure but it is possible to use class.

Closure

import functools


def cache(f):
    storage = {}

    @functools.wraps(f)
    def inner(n):
        value = storage.get(n)
        if value:
            print("Returning value from cache")
            return value
        value = f(n)
        storage[n] = value
        return value
    return inner


@cache
def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)

>>>factorial(20)
2432902008176640000
>>>factorial(20)
Returning from cache
2432902008176640000

cache is a function which takes function as an argument and returns a function.factorial is a function which calculates the factorial of a given number, decoratedby cache. If factorial of a number is calculated or less than already calculatednumber it is retrieved from storage.

The cache function can be written using Class.

Class

import functools


class Cache(object):
    def __init__(self, func):
        self.func = func
        self.storage = {}

        functools.update_wrapper(self, func)

    def __call__(self, n):
        value = self.storage.get(n)

        if value:
            print("Returning from cache")
            return value

        value = self.func(n)
        self.storage[n] = value
        return value


@Cache
def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)


>>>factorial(20)
2432902008176640000
>>>factorial(20)
Returning from cache
2432902008176640000

Cache class has two dunder methods, __init__, __call__. __init__ methodis called when interpreter encounters @Cache and __call__ is calledwhen factorial function is called.

See also

• Python Typing Koans
• Model Field - Django ORM Working - Part 2
• Structure - Django ORM Working - Part 1
• jut - render jupyter notebook in the terminal
• Five reasons to use Py.test