Improve Your Code with Decorators

How to create your own decorators in Python

Photo by Pakata Goh on Unsplash

In this article, we will see how powerful decorators are. If you are new to Python (and maybe even if you are not), you probably have never heard of this construct. It is something specific to Python, which allows defining some general behavior that is common to many methods in a separate function.

For example, suppose that we have the following methods:

def sum(a, b):
  return a+b
def subtract(a, b):
  return a-b

Now we want to print the result to the console, before returning it. What should we do? The first idea is to just add a print statement inside all functions. But what if then we decide to save it in a file instead? WE would need to change all the functions again.

Instead, we can create a decorator log_result and add it to the methods:

@log_result
def sum(a, b):
  return a+b
@log_result
def subtract(a, b):
  return a-b

And now if we want to change where we write the output, we just need to change the code inside log_result .

What is a decorator?

Now we have seen how decorators can be used, but what is a decorator exactly?

To give you a formal definition¹,

A decorator is a function that takes another function and extends the behavior of the latter function without explicitly modifying it.

Let’s break down this piece by piece.

First of all, a decorator is a function that takes another function as parameter: this means that we will define the log_result decorator as:

def log_result(func):

Then we need to modify this function and return the modified version.

So we can create an inner function (which I called inner for simplicity) which execute func , print out the result and finally return this value:

def log_result(func):
  def inner():
    res = func()
    print(res)
    return res

Finally, the decorator must return the modified function:

def log_result(func):
  def inner():
    res = func()
    print("The result is ", res)
    return res
  
  return inner

We have created our first decorator! Well, not really. It works only if the function we add it to has no parameters. For example:

@log_result
def get_even_digits():
  return [0, 2, 4, 6, 8]

When we call this function the log will print on the screen:

>>> l = get_even_digits()
The result is [0, 2, 4, 6, 8]

However, if we try to use it with the sum and subtract methods from before, the code will not run correctly.

The problem is that we haven’t given the modified function the ability to receive the parameters that should be passed to func . Luckily, this can be resolved easily. We will redefine inner as follows:

def log_result(func):
  def inner(*args, **kwargs):
    res = func(*args, **kwargs)
    print("The result is ", res)
    return res
  
  return inner

Note: if you don’t know what *args and **kwargs are, check this article.

Now innercan get any number of parameters, and pass them directly to the function. We have done it!

>>>s = sum(3, 5)
The result is 8
>>>d = subtract(8,5)
The result is 3

Decorators with parameters

Lastly, we may want to pass some additional parameters to the decorator. For example, we may want to print also a name together with the result, so that we know from which operation this log is coming from.

To do so, we need to modify slightly the function:

def log_with_name(name):
  def log_result(f):
    def inner(*args, **kwargs):
      res = f(*args, **kwargs)
      print(name + ": " + str(res))
      return inner
  return log_result

Let’s see what’s happening. The log_with_name function takes the name string and returns the decorator which will then be applied to a function.

It can be used exactly as before:

@log_with_name("Sum")
def sum(a, b):
  return a+b
@log_with_name("Difference")
def subtract(a, b):
  return a-b

And the result will be:

>>> s = sum(3, 5)
Sum: 8
>>> d = subtract(8, 3)
Difference: 5

Conclusion

Thank you for reading through the end! Here are some other resources on decorators:

Primer on Python Decorators - Real Python

PEP 318 -- Decorators for Functions and Methods

More content at plainenglish.io. Sign up for our free weekly newsletter. Get exclusive access to writing opportunities and advice in our community Discord.