Decorators In Python & How To Understand Them

# A step by step walkthrough

@something
def your_function(a, b):
  # stuff

^ You’ve probably seen something like this. The @something on top of our function is known as a decorator.

What The @ Means

Let’s say we have a simple function and a simple decorator.

@my_decorator
def my_function(x):
  # stuff

This is the exact same as the following code:

def my_function(x):
  # stuff

my_function = my_decorator(my_function)

The @ simply provides us a way to make our code look more pretty. But what it is actually doing is in the second code block.

What a Decorator is

A decorator is a function that:

1. Takes in a function
2. Returns another function

Decorators are used to change how a function behaves.

def greet(name):
  return 'hello ' + name

^ Here, we have a simple greet function.

def add_exclamation(function):
  def inner(*args):
    return function(*args) + '!'
  return inner

# ^ this function is a decorator function
# it adds one ! after its input function's return value

^ and here, we have a simple decorator. Remember, a decorator itself is a function. It just takes in a function, and returns another function.

def add_exclamation(function):
  def inner(*args):
    return function(*args) + '!'
  return inner

@add_exclamation
def greet(name):
  return 'hello ' + name

print(greet('tim'))  # hello tim!

^ Our decorator and decorated function in action.

@add_exclamation
@add_exclamation
def greet(name):
  return 'hello ' + name

print(greet('tim'))  # hello tim!!

^ Note that we can decorate a function more than once.

How This Works

@add_exclamation
def greet(name):
  return 'hello ' + name

# THIS IS THE SAME AS

greet = add_exclamation(greet)

def add_exclamation(function):
  def inner(*args):
    return function(*args) + '!'
  return inner

# Tracing add_exclamation(greet)

function = greet

# inner becomes
def inner(*args):
  return greet(*args) + '!'

  # here is where the ! is added

# inner is then returned

# Essentially

greet = inner

# where inner is

def inner(*args):
  return greet(*args) + '!'

And that is how decorators work on a very basic level.

Conclusion

Hope this was clear and helpful

Some Final words

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