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[Feature] Use typing.Annotated for customization

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难度
5/5
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一周以上
新手友好度
35/100
Issue 类型
功能
描述清晰度
基本清楚
活跃度
停滞
技术栈
python
领域
backend

调研方向

Review the existing converter-customization path and the proposed cattrs.annotations API, then compare it with typing.get_type_hints and the Annotated example in this issue. Done means the supported metadata behavior, backward-compatibility expectations, and implications for issues #352 and #552 are defined and verified.

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描述

  • cattrs version: 23.2.3
  • Python version: 3.12
  • Operating System: MacOS
Description

There's a decent amount of overhead in creating separate converters that needs to be done, which can get cumbersome quickly for a large number of classes. I was thinking it'd be possible to use typing.Annotated to reduce this work in a backwards-compatible way, without messing up type checking for those who need it.

An example that could solve (#352 and #552 if I understand them correctly)

from typing import Annotated
import cattrs.annotations as ca   # hypothetically

class SomeKVPair:
    key: Annotated[str, ca.Rename("Key")]
    data_size: Annotated[int, ca.FromNested("Data.Size")]
    value: Annotated[bytes, ca.AsBase64, ca.Rename("Value")] = b""

There are a few benefits to this:

  1. Type checkers will still understand the annotations.
  2. No need to rely on metadata inserted using attrs.field() or similar.
  3. It's clear to anyone looking at just the class what transforms are made. If they want to do their own serialization (e.g. create an XML serializer) it'll be clear what transforms they need to do.
  4. Getting the extra annotations is opt-in, so it'll be invisible to older versions of cattrs that don't support it.
  5. Other libraries can also see these and use it for their own purposes.
  6. It's been available since 3.9. Version 3.8 goes EOL in October, so there's no need for compatibility shims.

Presumably this will be a non-trivial change, but I think it would be really valuable.

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从这里开始

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  3. Fork 仓库,在一个分支上完成修改。
  4. 提交 Pull Request,并在描述里引用这个 Issue 编号。

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