Python Property Decorator
Summary: in this tutorial, you’ll learn about Python property decorator (@property) and more importantly how it works.
Introduction to the Python property decorator
In the previous tutorial, you learned how to use the property class to add a property to a class. Here’s the syntax of the property class:
class property(fget=None, fset=None, fdel=None, doc=None)Code language: Python (python)
The following defines a Person class with two attributes name and age:
class Person:
def __init__(self, name, age):
self.name = name
self.age = ageCode language: Python (python)
To define a getter for the age attribute, you use the property class like this:
class Person:
def __init__(self, name, age):
self.name = name
self._age = age def get_age(self):
return self._age
age = property(fget=get_age)
Code language: Python (python)
The property accepts a getter and returns a property object.
The following creates an instance of the Person class and get the value of the age property via the instance:
john = Person('John', 25)
print(john.age)Code language: Python (python)
Output:
25Code language: Python (python)
Also, you can call the get_age() method of the Person object directly like this:
print(john.get_age())Code language: Python (python)
So to get the age of a Person object, you can use either the age property or the get_age() method. This creates an unnecessary redundancy.
To avoid this redundancy, you can rename the get_age() method to the age() method like this:
class Person:
def __init__(self, name, age):
self.name = name
self._age = age def age(self):
return self._age
age = property(fget=age)
Code language: Python (python)
The property() accepts a callable (age) and returns a callable. Therefore, it is a decorator. Therefore, you can use the @property decorator to decorate the age() method as follows:
class Person:
def __init__(self, name, age):
self.name = name
self._age = age
def age(self):
return self._age
Code language: Python (python)
So by using the @property decorator, you can simplify the property definition for a class.
Setter decorators
The following adds a setter method (set_age) to assign a value to _age attribute to the Person class:
class Person:
def __init__(self, name, age):
self.name = name
self._age = age
def age(self):
return self._age
def set_age(self, value):
if value <= 0:
raise ValueError('The age must be positive')
self._age = value
Code language: Python (python)
To assign the set_age to the fset of the age property object, you call the setter() method of the age property object like the following:
class Person:
def __init__(self, name, age):
self.name = name
self._age = age
def age(self):
return self._age
def set_age(self, value):
if value <= 0:
raise ValueError('The age must be positive')
self._age = value
age = age.setter(set_age)
Code language: Python (python)
The setter() method accepts a callable and returns another callable (a property object). Therefore, you can use the decorator @age.setter for the set_age() method like this:
class Person:
def __init__(self, name, age):
self.name = name
self._age = age
def age(self):
return self._age
def set_age(self, value):
if value <= 0:
raise ValueError('The age must be positive')
self._age = value
Code language: Python (python)
Now, you can change the set_age() method to the age() method and use the age property in the __init__() method:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def age(self):
return self._age
def age(self, value):
if value <= 0:
raise ValueError('The age must be positive')
self._age = value
Code language: Python (python)
To summarize, you can use decorators to create a property using the following pattern:
class MyClass:
def __init__(self, attr):
self.prop = attr
def prop(self):
return self.__attr
def prop(self, value):
self.__attr = value
Code language: Python (python)
In this pattern, the __attr is the private attribute and prop is the property name.
The following example uses the @property decorators to create the name and age properties in the Person class:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def age(self):
return self._age
def age(self, value):
if value <= 0:
raise ValueError('The age must be positive')
self._age = value
def name(self):
return self._age
def name(self, value):
if value.strip() == '':
raise ValueError('The name cannot be empty')
self._age = value
Code language: Python (python)
Summary
- Use the
@propertydecorator to create a property for a class.