Making an auto-updating attribute

Let's talk about how to make an automatically updating attribute in Python.

Attributes don't change automatically

We have a Rectangle class:

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height
        self.area = self.width * self.height

When we call this class, we'll get a Rectangle object:

>>> rect = Rectangle(3, 4)

This Rectangle object has a width, a height, and an area:

>>> rect.width
3
>>> rect.height
4
>>> rect.area
12

If we're considering acceptable to change the width or height or an existing Rectangle object, we might have problem. If we change the width of a Rectangle the area won't change automatically:

>>> rect.width = 10
>>> rect.area
12

This happens because we only assigned the area one time: in our initializer method. When we first make a Rectangle object, we set the area in our __init__ method, and that's it.

We don't use getter methods

In a lot of programming languages, it's common to fix this problem by making a getter method. Instead of an area attribute we would make a method (maybe called get_area or just area):

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

When we access the area method now, we'll see that it's a bound method:

>>> rect = Rectangle(3, 4)
>>> rect.area
<bound method Rectangle.area of <__main__.Rectangle object at 0x7f028eb3caf0>>

We can't just access rect.area to get our area anymore. We have to call the area method in order to get a result:

>>> rect.area()
12

Now if we update the width of our Rectangle object:

>>> rect.width = 10

And call the area method:

>>> rect.area()
40

Python will recompute the area and give us back the correct result.

In Python, writing getter methods is not common. Instead of using getter method, we tend to make a property.

We prefer properties over getter methods

We're using the property decorator here:

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    @property
    def area(self):
        return self.width * self.height

That's called a decorator because of that @ syntax.

When we use the property decorator, it will make what was previously a method into a property.

Now if we make a Rectangle object, just like before, and we access the area attribute:

>>> rect = Rectangle(3, 4)
>>> rect.area
12

Unlike before, we don't need to put parentheses after area! In fact, we can't put parentheses. Simply by accessing the area attribute we get our answer.

So if we change the width or the height and we access area again, it will recompute the area automatically:

>>> rect.width = 10
>>> rect.area
40

So Python isn't actually storing the area attribute anywhere. Instead, every time the area attribute is accessed, it calls area method and gives us back whatever the return value is. And it does this automatically, simply because we accessed the rect.area attribute.

Use properties for auto-updating attributes

If you want to make an attribute on an object in Python that updates automatically you can use a property.