[PYTHON] I read PEP 560 (Core support for typing module and generic types)

Continuing from yesterday, when I was looking into the attribute __origin__, I found a PEP called PEP 560 --Core support for typing module and generic types. I arrived at. This PEP has been around since 3.7, so it's an implementation that already exists and can be touched.

Overview

• When the type hint came out, it was designed to keep the Python body untouched.
• However, there were three problems at that time (version 3.6).
• Typing module performance
• Metaclass conflict
• Numerous hacks used in the typing module

approach

• Add two special methods, __class_getitem__ and __mro_entries__
• __class_getitem__
• __Getitem__ for class objects
• Used in situations such as ʻIterable [int]`
• __mro_entries__
• Called when creating a derived class from an object that is not a class
• Determine the MRO of the derived class based on the class returned by __mro_entries__
• A mechanism to realize typing._GenericAlias
• __mro_entries__ ofList [int]returns list, so it behaves as a subclass of list at runtime.
• Inherited classes at class definition are stored in __orig_base__, and resolved inherited classes are stored in __base__

Impressions

• I read the PEP that triggered the birth of typing._GenericAlias
• It's helpful to read this PEP and then the typing module.
• The purpose of starting PEP was __origin__, which was a punch line that was a private attribute to realize __mro_entries__.
• If you use __class_getitem__ well, it will be worth hacking.