I read PEP 585 (Type Hinting Generics In Standard Collections)

When I was investigating where the attribute __origin__ came from, a PEP called PEP 585 --Type Hinting Generics In Standard Collections I found it, so I forgot the original purpose and skimmed it. I'm still in Draft status, so I'm not sure if it will be adopted in the future, but I'll make a note of my understanding.

Overview

• Previous type annotations used dedicated types to represent collections (such as typing.List and typing.Dict).
• Integrate collection implementation and typeclasses to allow standard collection classes (such as list and dict) for type annotation
• I want to abolish the generic collection type under typing

the term

• Generic
• Parameterizable type. Generally refers to a container.
• Also known as parametric type and generic type.
• Example: dict
• parametrized generic
• An instance of Generic with the type of container element
• Also known as parametrized type.
• Example: dict [str, int]

approach

Use list and dict instead of the previous typing.List and typing.Dict. that's all.

def find(haystack: dict[str, list[int]]) -> int:
    ...

What not to do

• Does not support ʻis instance (obj, list [int])`
• Does not support ʻissubclass (cls, list [str])`

Note

Internally, it is supposed to create an instance of types.GenericAlias. Therefore, you can use __origin__ or __args__ to retrieve type information.

StrList = list[str]

assert isinstance(StrList, types.GenericAlias)
assert StrList.__origin__ is list
assert StrList.__args__ is (str,)

Impressions

• It's not simple to have a type dedicated to annotations apart from the class you usually use, so this looks very good (although I'm used to it)
• It's hard to scream with old code when the generic type of typing is abolished (about 5 years later?)
• As the version of Python3 progresses, I feel that the type annotations are strongly tied to the Python language specification.
• The type checker is still outside, but when writing Python, it seems that the flow should be to write the shape ...
• I have a feeling that it will not stop once it is used in performance and testing

Postscript

Before I knew it, it was decided to adopt PEP 585 (Accepted), and it was available in 3.9.0a6. Below is the state of 3.9-dev installed a while ago.

$ python3.9
Python 3.9.0a6+ (heads/master:7f7e706, May  9 2020, 13:35:20)
[Clang 11.0.3 (clang-1103.0.32.59)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> list[int]
list[int]
>>> dict[str, int]
dict[str, int]
>>> tuple[int, ...]
tuple[int, ...]