@property decorator#

In Python, you can access instance variables directly, without additional getter and setter methods that are often used in Java and other object-oriented languages. This makes writing Python classes cleaner and easier, but in some situations using getter and setter methods can also be useful. Let’s say you need a value before setting it in an instance variable, or you just want to find out the value of an attribute. In both cases, getter and setter methods would do the job, but at the cost of losing easy access to instance variables in Python.

The answer is to use a property. This combines the ability to pass access to an instance variable via methods such as getters and setters with simple access to instance variables via dot notation. To create a property, the property decorator is used with a method that has the name of the property:

23    @property
24    def length(self):
25        return self.__length

Without a setter, however, the property length is read-only:

>>> s1 = form.Square()
>>> s1.length = 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

To change this, you need to add a setter:

27    @length.setter
28    def length(self, new_length):
29        self.__length = new_length

Now you can use the dot notation to both get and set the property length. Note that the name of the method remains the same, but the decorator changes to the property name, in our case to length.setter:

>>> s1 = form.Square()
>>> s1.length = 2
>>> s1.circumference()

A big advantage of Python’s ability to add properties is that you can work with plain old instance variables at the beginning of development and then seamlessly switch to property variables whenever and wherever you need to, without changing the client code. The access is still the same, using dot notation.