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Special handling of descriptor field like in dataclasses

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
25/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
tooling

Research direction

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.

Written by the indexing model from the issue text.

Description

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
Dominant language
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Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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