Special handling of descriptor field like in dataclasses

[getzze]

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

[hynek]

I was wondering when they added it, because I didn't have it on my radar at all, but it looks like 3.10 – if the docs are to be trusted (3.9 doesn't mention descriptors at all).

Does anyone have a clue how much work it would be to integrate this?

[getzze]

I checked the source code of `dataclasses` and the only mention of descriptors is in this line: https://github.com/python/cpython/blob/718dbd4cd33a204e12fb747a09b73c4c4ac1b55f/Lib/dataclasses.py#L349 Maybe defining the method `__set_name__` is enough to instantiate the descriptor and make it work.

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On 3 February 2024 08:02:38 GMT+01:00, Hynek Schlawack ***@***.***> wrote: I was wondering when they added it, because I didn't have it on my radar at all, but it looks like 3.10 – if the docs are to be trusted (3.9 doesn't mention descriptors at all). Does anyone have a clue how much work it would be to integrate this? -- Reply to this email directly or view it on GitHub: #1232 (comment) You are receiving this because you authored the thread. Message ID: ***@***.***>

[Tinche]

I'm in favor of fixing this on philosophical grounds (I'm bothered by dataclasses doing a thing better than us) ;)

[hynek]

Yes, it's very irritating.

[getzze]

Maybe it's not too hard to implement, because annotating the descriptor as `ClassVar` is enough to make it work: ```python # attrs: THIS WORKS from typing import ClassVar @define(slots=False) class InventoryItem3: quantity_on_hand: ClassVar[IntConversionDescriptor] = IntConversionDescriptor(default=100) i3 = InventoryItem3() print(i3.quantity_on_hand) # 100 i3.quantity_on_hand = 2.5 print(i3.quantity_on_hand) # 2 ```

…

On 4 February 2024 18:04:49 GMT+01:00, Hynek Schlawack ***@***.***> wrote: Yes, it's very irritating. -- Reply to this email directly or view it on GitHub: #1232 (comment) You are receiving this because you authored the thread. Message ID: ***@***.***>