Decorator in Python

Posted on August 19, 2017
Tags: programming python decorator

A decorator in a python is a function that adds some functionalities to an existing function without altering the function itself. That is, a wrapper to an existing function.

Why do we need decorator?

Sometimes it happens to us that we want to add some features to an existing one. Like there is some kind of library function that you are using it. However, you are not satisfied by its mechanism. So, you want to add some more features to it. But here’s the dilemma.

I. You want to alter the existing feature itself. But you have to touch the code in a third-party library.

II. You want to touch your old code. But you are afraid that you might brick the overall system in the process.

III. You can’t just keep on changing old code. You might have to do it at multiple locations where the old functions are being called.

Solution? Decorators

Decorator provides a means of altering or adding more code to an existing function. But you don’t really have to touch the old code. It’s like:

I. You take a function.

Let’s take our old function.

def old_function():
    pass

II. Do whatever you’d like to do it to that function.

Now let’s wrap it.

def wrapper(func):
    new_func = wrap(func)
    return new_func

III. Return the improved version of that function.

Finally, you can just alter old_function as:

old_function = wrapper(old_function)

Example : Pokemon

Say we have a function named pokemon that takes name and level of the pokemon.

def pokemon(name="Pikachu", level=5):
    return "{0} --> {1}".format(name, level)

Now, above function is called at multiple locations or modules say:

...
bulbasaur = pokemon("bulbasaur", 6)
# do something with bulbasaur
...
...
psyduck = pokemon("psyduck", 10)

Let pokemons get evolved
After some time, we want to be able to make pokemon evolve perhaps.
It can be done as:

...
bulbasaur = pokemon("bulbasaur", 6)
bulbasaur_evolved = pokemon("bulbasaur", 12)
...
...
psyduck = pokemon("psyduck", 10)
psyduck_evolved = pokemon("psyduck", 15)

Pretty annyoing, right?
We can’t just go to multiple locations and keep on making changes accordingly.

Oh wait! We can do it better

def evolve_pokemon(name="Pikachu", level=5):
    return "{0} --> {1}".format(name, level*2)

But this is rubbish. We don’t want to make changes to original code.

Here comes our hero decorator
Now, let’s create a new function evolve that accepts the original function pokemon and wraps with evolution

EVOLUTION_MULTIPLIER = 2

def evolve(pokemon):
    def wrap(name, level):
        return pokemon(name, level * EVOLUTION_MULTIPLIER)
    return wrap

@evolve
def pokemon(name="Pikachu", level=5):
    return "{0} --> {1}".format(name, level)

The function evolve is now a decorator. The decorator then can be used using the @ symbol with the function name.
You should realize that the decorator evolve has an inner function wrap that basically performs the wrapping operation. wrap returns a wrapped function. And evolve in turn returns its inner function wrap.

This is the fundamental principle of creating a decorator in python: function inside function.

Hey! I want a decorator that accepts evolution similar to calling a function like @evolve(3).
No problem mate. This can be done by wrapping the evolve with yet another function.

def evolve(n):
    def inner(pokemon):
        def wrap(name, level):
            return pokemon(name, level * n)
        return wrap
    return inner

@evolve(3)
def pokemon(name="Pikachu", level=5):
    return "{0} --> {1}".format(name, level)

Did you notice anything? We took the first version of the decorator evolve and renamed it to inner. And then we wrapped it with new function evolve that takes the numerical value n.

You are seeing the pattern, right?

Function inside a function inside a function inside a function…..

You can basically take it to any level. But beyond 3 wrappers, the decorator doesn’t really makes sense in terms of usecase.
Decorators are possible in python because a function itself is an object in python and you can pass it around, kick its butt and play around.