A python decorator allows you to remove repeated code by passing function into another function which extends it.

• An Example
• Practical applications
• Decorator Factories

An Example

Here is an example of a decorator:

def printer(func):
    def inner(*s):
        print(*s)
        return func(*s)
    return inner

@printer
def add(a, b):
    return a + b

if __name__ == "__main__":
    print(add(1, 2))

Here is what it would look like without it:

def printer(func):
    def inner(*s):
        print(*s)
        return func(*s)
    return inner

def add(a, b):
    return a + b
add = printer(add)

if __name__ == "__main__":
    print(add(1, 2))

Practical applications

Measuring the time a function takes to run is one of many uses for a decorator, here is how it's done:

import time

def measure_time(func):
    def wrapper(*arg):
        t = time.time()
        res = func(*arg)
        print("Function took " + str(time.time()-t) + " seconds to run")
        return res
    return wrapper

@measure_time
def myFunction(n):
    time.sleep(n)

if __name__ == "__main__":
    myFunction(2)

Another use is in exposing a function to something, here is an example from flask:

from flask import Flask

app = Flask(__name__, instance_relative_config=True)

# a simple page that says hello
@app.route('/hello')
def hello():
    return 'Hello, World!'

Decorator Factories

A decorator factory is a function that takes arguments and returns an augmented decorator with it, here is an example:

def printfunc(s):
    def wrapper(func):
        def inner(*arg):
            o = func(*arg)
            print(s, o)
            return o
        return inner
    return wrapper

@printfunc("add:")
def add(a, b):
    return a + b

if __name__ == "__main__":
    add(1, 2)