How to use descriptors in Python

Overview

A descriptor is a Python object that defines the operations to be performed when accessing the properties of other objects. Through descriptors, you can implement a variety of different behaviors, such as calculating properties, caching property values, and controlling property access. Use descriptors to customize property access behavior and avoid writing duplicate code for each property use.

Attributes of any class, including instance attributes, class attributes and static attributes, can use descriptors. Descriptors in Python programming are advanced features that are very useful for programmers with in-depth understanding of the Python language and advanced programming skills.

Implementation method

Python descriptors are defined by implementing the descriptor protocol. The descriptor protocol is a type of Python object protocol, which defines three methods: __get__(), __set__(), and __delete__().

When the Python interpreter accesses the properties of an object, it will first check whether the property is a descriptor. If the attribute is a descriptor, call the __get__() method to obtain the attribute value. If the attribute is not a descriptor, the attribute value is returned directly.

If we want to use a Python descriptor to control attribute access behavior, we need to implement __get__(), __set__() and # in the descriptor protocol ##__delete__()At least one method in the method. The following is the specific description of these methods:

__get__(self, instance, owner): used to obtain attribute values. If the attribute being accessed is an instance, the instance parameter is the instance object and the owner parameter is the class object. If the attribute being accessed is a class, the instance parameter is None and the owner parameter is the class object.

__set__(self, instance, value): Used to set attribute values. If the attribute value is set by an instance, the instance parameter is the instance object and the value parameter is the value to be set. If the attribute value is set by a class, the instance parameter is None and the value parameter is the value to be set.

__delete__(self, instance): Used to delete attribute values. If the property value being deleted is an instance, the instance parameter is the instance object. If the attribute value is being deleted from a class, the instance parameter is None. How to use Python descriptors

Application scenarios

Python's descriptors can be used in a variety of situations, such as calculating attributes, caching attribute values, and implementing attribute access control. Here are some examples using Python descriptors.

Computed property

A calculated property is a property calculated from other properties. For example, using a descriptor you can create a computed property that adds two numeric properties. The following is a sample code that implements a calculated property:

class SumDescriptor:  
    def __init__(self, a, b):  
        self.a = a  
        self.b = b  
     
    def __get__(self, instance, owner):  
        return getattr(instance, self.a) + getattr(instance, self.b)  
     
class MyClass:  
    def __init__(self, a, b):  
        self.a = a  
        self.b = b  
        self.sum = SumDescriptor('a', 'b')

In the above code, SumDescriptor is a descriptor that uses the __get__() method to calculate the sum of the a and b attributes. MyClass is a class that contains properties a and b. It also defines a sum property, which is an instance of SumDescriptor.

When we create an instance using MyClass, we can get the sum of the a and b attributes by accessing the sum attribute without having to calculate them manually:

>>> obj = MyClass(1, 2)  
>>> obj.sum  
3

Cache attribute values

Another common use is caching attribute values. Using descriptors allows you to cache attribute values, thereby improving program performance, especially when the attribute value is a slow calculation or a large amount of data. The following is a sample code for caching property values:

class CachedProperty:  
    def __init__(self, func):  
        self.func = func  
        self.__name__ = func.__name__  
         
    def __get__(self, instance, owner):  
        if instance is None:  
            return self  
        value = self.func(instance)  
        setattr(instance, self.__name__, value)  
        return value  
  
class MyClass:  
    def __init__(self, data):  
        self._data = data  
         
    @CachedProperty  
    def processed_data(self):  
        # Perform some slow computation  
        result = ...  
        return result

In the above code,

CachedProperty is a descriptor that uses the __get__() method to cache properties value. MyClass is a class containing _data attributes, which defines a processed_data attribute, which uses the @CachedProperty decorator to implement caching . When we access the processed_data attribute, if the attribute value already exists in the cache, the cached value will be returned directly. Otherwise, the property value is calculated and stored in the cache.

Implementing attribute access control

Descriptors can also be used to implement attribute access control. For example, we can use descriptors to disable modifications to a property. The following is a sample code that implements attribute access control:

class ReadOnlyDescriptor:  
    def __init__(self, value):  
        self.value = value  
         
    def __get__(self, instance, owner):  
        return self.value  
     
    def __set__(self, instance, value):  
        raise AttributeError("can't set attribute")  
  
class MyClass:  
    def __init__(self, data):  
        self._data = ReadOnlyDescriptor(data)

In the above code,

ReadOnlyDescriptor is a descriptor that uses the __set__() method to prohibit Make modifications to properties. MyClass is a class containing the _data attribute, which defines a read-only attribute. When we try to modify the _data attribute, an AttributeError exception will be thrown.

Custom property access control

In addition to the basic descriptors introduced above, Python also provides

property decorators, which can be used to define custom property access control . Using the property decorator, we can convert a method into a read-only property, a writable property, or a read-write property. The following is a sample code for custom attribute access control:

class MyClass:  
    def __init__(self, value):  
        self._value = value  
         
    @property  
    def value(self):  
        return self._value  
     
    @value.setter  
    def value(self, new_value):  
        if new_value < 0:  
            raise ValueError("value must be non-negative")  
        self._value = new_value

在上面的代码中，value方法被转换为一个属性。@property装饰器将value方法转换为只读属性，@value.setter装饰器将value方法转换为可写属性。当我们尝试对value属性进行修改时，如果新值小于0，则引发ValueError异常。

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