Python Descriptor (Descriptor)

Brief introduction:

A Python descriptor is a language protocol in a modern class (inherited from Object) that provides a better and more elegant solution based on descriptors.

Python's Classmethod, Staticmethod, and property are all built on descriptors.

Descriptor's protocol:

An object that defines __set__, __get__, __delete__3 any of the methods in a method can be used as a descriptor.

Descriptor Categories:

Also defined __set__,__get__ is called data descriptor.

Only the definition of __get__ is called No-data descriptor.

2 different descriptors:

Data and non-data descriptors differ in how overrides is calculated with respect to entries in an instance ' s dictionary. If An instance ' s dictionary have a entry with the same name as a data descriptor, the data descriptor takes precedence. If An instance ' s dictionary have an entry with the same name as a Non-data descriptor, the dictionary entry takes Precedenc E.

In the Attrubuite lookup process:

If an object property has an attribute with the same name as Data-descriptor, Data-descriptor takes precedence over the object property.

If an object property has an attribute with the same name as No-data descriptor, the object property takes precedence.

Call to trigger Descriptor:

A descriptor can called directly by its method name. For example, d.__get__(obj) .

Alternatively, it's more common for a descriptor to be invoked automatically upon attribute access. For Example, OBJ.D  looks Up d  in the dictionary of  obj . If d  defines the Method __get__ () , Then< Span class= "Pre" >d.__get__ (obj)  is invoked according to the precedence rules listed below.

The details of invocation depend on Whether obj  is An object or a class. Either, descriptors only work for new style objects and classes. A class is new style if it is a subclass Of  Object .

For objects, the machinery are in object.__getattribute__() which transforms b.x into type(b).__dict__[‘x‘].__get__(b, type(b)) . The implementation works through a precedence chain that gives data descriptors priority over instance variables, instance Variables priority over Non-data descriptors, and assigns lowest precedence to __getattr__() if provided

The important points to remember is:

• Descriptors is invoked by the __getattribute__() method
• overriding __getattribute__() prevents automatic descriptor calls
• __getattribute__()IS-available with new style classes and objects
• object.__getattribute__()And make type.__getattribute__() different calls to __get__() .
• Data descriptors always override instance dictionaries.
• Non-data descriptors May is overridden by instance dictionaries.

Python Descriptor (Descriptor)