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Special handling of descriptor field like in dataclasses

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技术栈
python
领域
tooling

调研方向

Start by reading the descriptor-typed fields documentation linked in the issue, then review the earlier discussions in #881 and #1108. Done means attrs supports descriptor fields without losing descriptor behavior during initialization or assignment, matching the documented dataclasses example.

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描述

dataclasses Feature

dataclass dataclasses handle descriptor fields differently from other fields, so the descriptor type is not lost when assigning:
https://docs.python.org/3/library/dataclasses.html#descriptor-typed-fields

This does not work in attrs, it's a pity because it could be used as an alternative to converters called with the instance (#1108).
Previous discussion about this was not very conclusive (#881).

# Example from python docs
from dataclasses import dataclass
from attrs import define


class IntConversionDescriptor:
    def __init__(self, *, default):
        self._default = default

    def __set_name__(self, owner, name):
        self._name = "_" + name

    def __get__(self, obj, type):
        if obj is None:
            return self._default

        return getattr(obj, self._name, self._default)

    def __set__(self, obj, value):
        setattr(obj, self._name, int(value))

# dataclass: WORKS AS EXPECTED
@dataclass
class InventoryItem:
    quantity_on_hand: IntConversionDescriptor = IntConversionDescriptor(default=100)

i = InventoryItem()
print(i.quantity_on_hand)   # 100
i.quantity_on_hand = 2.5    # calls __set__ with 2.5
print(i.quantity_on_hand)   # 2


# attrs: DOES NOT WORK
@define
class InventoryItem2:
    quanttity_on_hand: IntConversionDescriptor = IntConversionDescriptor(default=100)

i2 = InventoryItem2()
print(i2.quantity_on_hand)   # <__main__.IntConversionDescriptor object at 0x78c3a12a1250>
i2.quantity_on_hand = 2.5    # set InventoryItem2 attribute to a float, erasing the descriptor
print(i2.quantity_on_hand)   # 2.5
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环境准备

从这里开始

  1. 先读完整个 Issue，再读项目的贡献指南。
  2. 在 Issue 下留言说明你要接手 —— 这能避免两个人做同样的事。
  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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