slotted dataclasses not calling descriptor field `__set__`

[thejcannon]

Bug report

Bug description:

(Sorry to be finding these funny little dudes)

import dataclasses


class DescriptorFieldType:
    def __set_name__(self, owner, name: str) -> None:
        self.name = name

    def __get__(self, instance, owner):
        if instance is None:
            return None
        return instance.__dict__[self.name]

    def __set__(self, instance, value) -> None:
        raise ValueError("Unsettable")


@dataclasses.dataclass(slots=True)
class Slotted:
    field: DescriptorFieldType = DescriptorFieldType()

Slotted()

@dataclasses.dataclass
class Unslotted:
    field: DescriptorFieldType = DescriptorFieldType()

Unslotted()

Will error with:

Traceback (most recent call last):
  File "[...]", line 27, in <module>
    Unslotted()
  File "<string>", line 3, in __init__
  File "[...]", line 14, in __set__
    raise ValueError("Unsettable")
ValueError: Unsettable

(errors on Unslotted but not Slotted)

CPython versions tested on:

3.11

Operating systems tested on:

No response

Linked PRs

• gh-132946: Do not allow setting data descriptors in @dataclass(slots=True) #133002

[picnixz]

cc @ericvsmith

[sobolevn]

My wild guess: maybe this happens because we recreate Slotted class?
It would be really interesting to find out. Thanks a lot!

[thejcannon]

Only the finest issues for my two favorite python features 🙃

[ericvsmith]

From https://docs.python.org/2/reference/datamodel.html?highlight=__slots__#__slots__:

slots are implemented at the class level by creating descriptors (Implementing Descriptors) for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by slots; otherwise, the class attribute would overwrite the descriptor assignment.

It might be possible to detect this and raise an error, if someone wants to look into it.

[sobolevn]

I found one more problem related to descriptors in @dataclass:

class DescriptorFieldType:
    def __get__(self, instance, owner):
        print(instance, owner)
        if instance is None:
            return 1
        return 2

@dataclass
class Regular:
    one: DescriptorFieldType = DescriptorFieldType()

assert Regular.one == 1
assert Regular().one == 2  # fails, is actually `1`

I propose to discus this in a separate issue.

[thejcannon]

I tried to consider if dataclasses could then wrap/replace the slots descriptors in custom descriptors that called into the user provided one, but:

• oof, descriptors on descriptors
• that still wouldn't work since the user descriptor wouldn't be able to set or get attributes

Since this isn't a dataclasses bug, feel free to close. (The alternative being raise an error as @ericvsmith suggests)

[thejcannon]

Ah, wait I think raising an error is the right way to go:

# This declaration doesn't error
@dataclass(slots=True)
class SlottedDataclass:
    name: Descriptor = Descriptor()

# This declaration errors
class Slotted:
    __slots__ = ("name",)

    name = Descriptor()