[PYTHON] I read PEP-593 (Flexible function and variable annotations)

I decided to read PEP-593 (Flexible function and variable annotations) in the flow of a discussion, so make a note of my understanding. Leave it in.

Overview

• PEP-3107 (Function Annotations) introduced annotations into the Python grammar. At that time, the uses of annotations were type hints, DB query mapping, RPS marshalling information, and so on.
• PEP-484 (Type Hints) introduced a way to implement type hints using annotation notation. It can now be said that it is the main usage of annotations.
• Now that PEP-484 has become the de facto standard, it has become difficult to use annotations for purposes other than type hints.
• So I'll redefine the annotations so that I can write my own metadata in addition to the type hints.

approach

• Add a new typing.Annotated
• Write ʻAnnotated [T, x, y]` when annotating a data or function with the type T and metadata x, y.
• The first element is considered a type
• Type checkers can refer to T, and tools and libraries that use metadata can refer to x and y.
• In other words, tools and libraries will ignore annotations that they don't support.

Example

Numerical (fictitious) annotations in the range 3-10 that are considered characters by ctype.

Annotated[int, ValueRange(3, 10), ctype("char")]

Impressions

• ʻAnnotated` is hard to write. I wanted to write more roughly in tuple ...
• You can get the contents of annotations even at runtime (get_type_hints ()), so it seems that libraries and frameworks that use this will come out.
• I hope it doesn't result in chaotic code as a result of abuse
• Although PEP-3107 (function annotation) is introduced, PEP-526 (Syntax for Variable Annotations) Is not mentioned at all. A pitiful child ...
• Of course, you can annotate variables as well as functions.
• Sphinx also has to support ...
• It seems that you can annotate the explanations of arguments and variables, but I feel that it is not popular because it is tedious to write anyway.
• I made an issue. I'm sure I will do it tomorrow ...

def hello(name: Annotated[str, Description("name")],
          message: Annotated[str, Description("Message format")],
          language: Annotated[str, Description("language")] = "ja"
          ) -> Annotated[None, Description("None")]:
    ...