Understand the Property Decorator of Python Classes

Introduction

In object-oriented programming, each attribute of a class may have three basic methods:

1. A getter method to get its value
2. A setter method to set its value
3. A deleter method to delete it

In Python, we can operate the attribute directly for the above three purposes:

However, life is not as easy as the above example. How can we deal with the wrong input like Yang._score = -10 or Yang._score = 99999 ?

A good idea is writing a setter function for the self._score to check the input value.

It looks good, but we can still set a invalid value directly:

If we use the set_score() function, the validity of the value will be verified. But if we forget using it and still setting values directly, no validation can be ensured.

Therefore, we still can’t totally ensure the value of self._score is always valid. Not to mention setting a value by a method is not as convenient as setting it directly. This is where the property decorator comes to rescue.

How to Use the Property Decorator?

Let’s modify the Student class like this:

As the above code shown, after using the @property decorator, we can both set the value directly and ensure the validity of it. The property decorator is a very useful and widely used mechanism in Python classes. It helps us call the getter, setter and deleter methods directly by the attribute.

A common template for using the property decorator is:

Note: The names of getter, setter and deleter methods should be the same. The best choice is to use the attribute’s name.

Another Way to Use the Property Decorator

The property is just a build-in decorator in Python. Therefore, like other decorators, we can use it by another way:

The property(fget=None,fset=None,fdel=None,doc=None) function has four parameters:

• fget() — used to get the attribute value
• fset() — used to set attribute value
• fdel() — used to delete the attribute value
• doc() — string that contains the documentation (docstring) for the attribute

Define a Read-only Attribute

As we known, there are three methods (getter, setter and deleter) we can define by the property decorator. If one of the three methods is not defined, we cannot use this method. Therefore, we can skillfully use the property decorator to define a read-only attribute. For example:

As the above example shown, we don’t define the setter method of score , so score can’t be set. In other words, it’s a read-only attribute. This is an important usage scenario of the property decorator.

Conclusion

The property decorator is a powerful tool and widely used in Python classes. It allows us to write concise code while ensuring the necessary checks on parameters, so that the possibility of errors is reduced.

Thanks for reading. More Python tutorials: