decorators.Rmd

---
title: "decorators"
date: 2018-08-12T15:30:35+02:00
draft: false
categories: ["Python"]
tags: ["Python", "decorators"]
---

## 1. What are decorators and why would you use them?

* decorators in Python are special functions that take a function as an argument and slightly change it's behaviour, e.g. it's return value;

* you can write your own decorators, which is rather easy (I highly recommend [Fluent Python](http://shop.oreilly.com/product/0636920032519.do) as a reference)

* but there are already many useful decorators available in Python.

I am not going to describe here how to write your own decorator as, to be honest, I used them only twice in my career. In fact, I didn't have to do that, I just wanted to try them out ;)

## 2. Useful built-in decorators

### @property

#### Why would you use property?

You crate a class with a specific attribute, say, `name`. At the beginning you are happy to use it only as:
```
object.name = 'Tomek'
print(object.name)
```

but after a while you realise that you need something else happening during setting a name and getting a name, something in the background, e.g. printing *you've just set a name* and *you've just got a name*. This would be easy if you had defined separate `getters` and `setters`, but you didn't and there are folks who already use this class, so they wouldn't be happy if they had to change their code. 

So you're on your own. Here's how you solve this problem:

```{python, engine.path = '/usr/bin/python3'}
class SomeFolk:

    def __init__(self, name):
        self._name = name

    @property
    def name(self):
        """this is just our previous, default behaviour"""
        return self._name

    @name.setter
    def name(self, name):
        """this is our brand new setter"""
        self._name = name
        print("you've just set a name to {}".format(self._name))  # log message

    @name.getter
    def name(self):
        """this is our brand new getter"""
        print("you've just got a name")  # log message
        return self._name
```

Let's create an instance of this class:
```{python, engine.path = '/usr/bin/python3'}
me = SomeFolk('Tomek')
```

Here's what happens when you get a name:
```{python, engine.path = '/usr/bin/python3'}
_some_variable = me.name
```

And when you set a name:
```{python, engine.path = '/usr/bin/python3'}
me.name = "tomis9"
```

Magic.

### @classmethod and @staticmethod

Here's a great tutorial describing the difference and usage:

<iframe width="853" height="480" src="https://www.youtube.com/embed/rq8cL2XMM5M" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

## 3. TODO

* an example of a self-made decorator