How Does Python's @property Decorator Work to Create Properties?

Understanding the @property Decorator in Python

In Python, the @property decorator allows you to create properties that behave like regular instance attributes. However, unlike built-in property functions that take arguments, the @property decorator does not explicitly receive any arguments.

How It Works

To understand how the @property decorator works, it's important to first understand that the property() function returns a descriptor object. This is an intermediary that sits between an instance and its property, allowing for additional logic to be applied.

The Descriptor Object

The descriptor object returned by the property() function has three built-in methods:

• getter: To retrieve the property value
• setter: To set the property value
• deleter: To delete the property

Creating Properties with the Decorator

When the @property decorator is applied to a function, it returns a descriptor object and assigns it to the property name. This object retains the getter function of the original function and includes two additional functions that delegate to the setter and deleter methods of the property.

For example:

class C:
    def __init__(self):
        self._x = None

    @property
    def x(self):
        """I'm the 'x' property."""
        return self._x

The above code creates a property named "x" that can retrieve the value of self._x without any explicit property object. The getter function is automatically created by the decorator.

Setter and Deleter Decorators

To add setter and deleter methods, simply append .setter and .deleter to the @property descriptor object, passing your intended functions:

@x.setter
def x(self, value):
    self._x = value

@x.deleter
def x(self):
    del self._x

By using these methods, you can implement custom behavior for setting and deleting the property value.

Implementation Details

The @decorator syntax for @property is syntactic sugar. Behind the scenes, the following code is executed:

def x(self): return self._x
x = property(x)

Descriptor Example

Here's an example of how a pure Python implementation of a property descriptor would work:

class Property:
    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        self.__doc__ = doc

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        if self.fget is None:
            raise AttributeError("unreadable attribute")
        return self.fget(obj)

This class allows you to manually create properties with getter, setter, and deleter functions.

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