Python Decorator with sample

Decorator is one of the powerful tool in meta programming . in python it is very simple to define and use a decorator

1. Function level decorator

Sample #1 function execution time

import time, random
def exe_time(func):
    print('before declaration...')
    def inner_func(*arg, **kwargs):
        arg_str = ','.join(list(map(str,arg)))
        print(f'===args: {arg_str}===')
        
        print(f'==keywarded args ==')
        for k,v in kwargs.items():
            print(f'{k}:{v}')
        start = time.time()
        print('before executing...')
        res = func(*arg, **kwargs)
        end=time.time()
        total = round(end-start,3)
        print(f'after executing...total {total}')
        return res
    print('after declaration...')
    return inner_func
@exe_time
def test(n,n2,p1='a',p2='b'):
    res=0
    for i in range(n):
        wait = random.randint(0,2)*i
        res+=wait
        time.sleep(wait)
    print(f'total wait :{round(res,2)} seconds')
    return res

Sample #2 Create a simple memory cache

import functools
import math
def cache_res(func):
    def inner(*arg, **kwarg):
        key = arg+tuple(kwarg.items())
        if key not in inner.cache:
            inner.cache[key] = func(*arg, **kwarg)
        return inner.cache[key]
    inner.cache = {}
    return inner
@cache_res
def test_compute(n):
    res= math.sqrt(n)+ math.sin(n) * math.cos(n)
    print(f'calculated result of :{n} -- {res}')
    return res
"""
    only calculated 2 times 
"""
if __name__ == "__main__":
    print(test_compute(10))
    print(test_compute(10))
    print(test_compute(5))
    print(test_compute(5))
    print(test_compute(5))

Class Level decorator

Sample#2 “template” id class field

import uuid
def add_id(cls):
    init = cls.__init__
    def my_init(self, *args, **kwargs):
        self.id = uuid.uuid1()
        init(self, *args, **kwargs)
    cls.__init__ = my_init
    return cls
@add_id
class person:
    def __init__(self,name):
        self.name=name
    def __str__(self):
        return f'{self.id},{self.name}'
def test_add_id():
    p = person('jeo')
    print(p)
if __name__ == "__main__":
    test_add_id()

Sample#2 Call counter

import functools
class call_counter:
    def __init__(self, f):
        functools.update_wrapper(self, f)
        self.func = f
        self.num = 0
    def __call__(self, *args, **kwargs):
        self.num +=1
        print(f'called {self.num} times')
        return self.func(*args, **kwargs)
@call_counter
def test_call():
    print("testing")
if __name__ == "__main__":
    test_call()
    test_call()
    test_call()

Sample #3 Simple flask auth

import functools
from flask import Flask
from flask import request
class auth:
    __name__ = "auth"
    def __init__(self, f):
        self.tokens = ["12345"]
    def __call__(self, *args, **kwargs):
        t = request.headers.get('token')
        print(t)
        if t not in self.tokens:
            return 'Unauthenticated'
        return self.func(*args, **kwargs)
app = Flask(__name__)
@app.route('/hello')
def helloIndex():
    return 'Hello World from Python Flask!'
@app.route('/auth_hello')
@auth
def auth_hello():
    return 'Hello with token'
app.run(host='0.0.0.0', port=5000)

Browse http://localhost:5000/auth_hello without sending token

Send request with correct token will see hello message .

Sample4 pass parameters to decorator

from functools import wraps
def post_log(level, name, message, save_result):
    def inner_func(func):
        @wraps(func)
        def wrapper(*arg, **kwargs):
            res = func(*arg, **kwargs)
            res_str = res if save_result else ''
            print(f'{name},{message} {res_str}')
            return res
        return wrapper
    return inner_func
@post_log("DEBUG", "name", "message", False)
def func1():
    return 1
@post_log("INFO", "name", "message", True)
def func2():
    return 2
if __name__ == "__main__":
    print(func1())
    print(func2())