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Getting Started: Python Decorators

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Ritesh Agrawal
·Dec 23, 2016·

6 min read

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This post will help you in getting started with Python decorators through some real life examples. Some familiarity with Python language is expected.

Before directly jumping into decorators, let’s take a step back and start with python functions. This will help in understanding the concept in a better way.

Functions

A function in python can be defined as follows:

def introduce(name):
    return 'My name is %s' % name

This function takes name as input and return a string, where:

  • def is the keyword used to define a function.
  • introduce is the name of the function.
  • Variable inside parenthesis (name) is the required argument for the function.
  • Next line is the body or definition of the function.

Function Properties

In python, functions are treated as first-class objects. This means that python treats functions as values. We can assign function to a variable, pass as an argument to a function or return as a value from a function.

def print_hello_world():
    print('Hello World!')

We have defined a function `print_hello_world'. Now we can assign it to a variable.

>>> modified_world = print_hello_world

(Here >>> is denoting the python interactive shell)

Now we can call modified_world just like the function print_hello_world.

>>>modified_world()
Hello World!

We can also pass a function to another function as an argument.

def execute(func):
    print('Before execution')
    func()
    print('After execution')

So now when we pass print_hello_world function to execute function, the output will be as follows:

>>>execute(print_hello_world)
Before execution
Hello World!
After execution

Python also supports nesting of functions. It means we can define another function in body or definition of some other function. Example:

def foo(x):
    def bar(y):
        return x+y
    return bar

In the above example, we have used two concepts described before.

  1. Returning a function (bar) as a return value of the function foo.
  2. Nesting function bar in the definition of the function foo.

Let's see this code in action.

>>>v1 = foo(2)

Here v1 stores the return value of the function foo which is another function bar. Now what will happen if we call v1 with some parameter?

>>>print(v1(5))
7

When a function is handled as data (in our case, return as a value from another function), it implicitly carries information required to execute the function. This is called closures in python. We can check the closure of the function using __closure__ attribute of the function. This will return a tuple containing all the closures of the function. If you want to see any content of the closure, you can do something like v1.__closure__[0].cell_contents.

>>>v1.__closure__
(<cell at 0x7f4368e6c590: int object at 0xa41140>,)
>>>v1.__closure__[0].cell_contents
2

So now, we understood both function properties. Let's see how can we use these properties in real scenarios.

Going Ahead

Suppose we want to perform some generic functionality before or/and after function execution. It can be like printing execution time of the function.

One way to do this is by writing whatever we want to do before and after execution as initial and final statements respectively. Example:

def print_hello_world():
    print('Before Execution')
    print('Hello World!')
    print('After Execution')

Is this a good way. I leave it to you. What will happen if I have several functions and need to perform the same task for all other functions too?

Another way could be to write a function that will take any other function as an argument and return the function along with performing the task before and after function execution. Example:

def print_hello_world():
    print('Hello World')

def dec(func):
    def nest_func(*args, **kwargs):
        print('Before Execution')
        r =  func(*args, **kwargs)
        print('After Execution')
        return r
    return nest_func

Function print_hello_world just prints 'Hello World'. Function dec takes a function as an argument, creates another function nest_func in its definition. nest_func prints some statements before and after execution of function passed as an argument in function dec.

Let's pass the function print_hello_world to dec.

>>> new_print_hello_world = dec(print_hello_world)

new_print_function is another function returned by the function dec. What will be the output on calling new_print_hello_world function? Let's check it.

>>> new_print_hello_world()
Before Execution
Hello World
After Execution

What if, we assign the new function returned by the dec function to print_hello_world function again?

>>> print_hello_world = dec(print_hello_world)

Let's call print_hello_world function now.

>>> print_hello_world()
Before Execution
Hello World!
After Execution

So we basically we have changed the functionality of the function print_hello_world without changing the source code of the function itself.

So What next? If are clear till now, then you have already learnt about decorators. Let me explain explicitly.

Decorators

A decorator is a function which provides us the freedom to enhance or change the functionality of any function dynamically, without making changes in the code of the function.

In our case, function dec provides us this functionality (As it changes the functionality of the function print_hello_world). So dec is called as decorator. Instead of passing print_hello_world explicitly to function dec, we can use its shorthand syntax:

@dec
def print_hello_world():
    print('Hello World')

I hope by now we are clear about decorators. You might be wondering why do I need to return a function from the dec function? Just call the function in dec itself in which we can print statements along with executing the function passed as argument. Example:

def dec1(func):
    print('Before Execution')
    func()
    print('After Execution')

I have few questions for you in answer to this question. Suppose I agree with you and decides to do it as suggested.

>>>print_hello_world = dec1(print_hello_world)
  1. What value does print_hello_world store right now? Can you call it now? (It is storing None which is return value of the function dec1. So you can't call print_hello_world now)
  2. What if we want to enhance function having some arguments. One suggestion could be like
def dec2(func, arg1, arg2):
    print('Before Execution')
    func(arg1, arg2)
    print('After Execution')

But here the problem is how will we get the value of arg1 and arg2 at time of passing any function to dec2.

>>> print_hello_world = dec2(print_hello_world, arg1, arg2)

Here we will not be able to get the value of arg1 and arg2.

I hope above two points have cleared your doubts about why decorators are required to return a function.

Decorator Examples

  • It can be used to compute execution time of any function.
def compute_execution_time(func):
    def nest_func(*args, **kwargs):
        start = time.time()
        response = func(*args, **kwargs)
        end = time.time()
        print(end-start)
        return response
    return nest_func
  • In web applications, it can be used to check if user is logged in or not.
def login_required(func):
    def nest_function(request, *args, **kwargs):
        if request.user.is_authenticated():
            return func(request, *args, **kwargs)
        else
            return redirect('/login')
    return nest_function

I hope this article provided you a basic idea of python decorators and some of its use cases.

 
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