| msg314077 - (view) |
Author: Adrian Stachlewski (stachel) |
Date: 2018-03-19 03:30 |
| I've tried to declare two classes @dataclass class Base: __slots__ = ('x',) x: Any @dataclass class Derived(Base): x: int y: int As long as I correctly understood PEP 557 (inheritance part), changing type of variable is possible. This code produce error: TypeError: non-default argument 'y' follows default argument 'x' variable in Derived class has changed default from MISSING to member_descriptor and that's the reason of the exception. |
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| msg314087 - (view) |
Author: Eric V. Smith (eric.smith) *  |
Date: 2018-03-19 08:57 |
| This is the same reason that this fails: class Base: __slots__ = ('x',) x = 3 with: ValueError: 'x' in __slots__ conflicts with class variable In the dataclasses case, the error needs to be improved, and moved to when the base class is being defined. |
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| msg314090 - (view) |
Author: Adrian Stachlewski (stachel) |
Date: 2018-03-19 09:14 |
| I don't really get your point. @dataclass class Base: __slots__ = ('x',) x: Any This case is described in PEP 557 as correct, so I don't understand why you want to generate error. Also inheritance without defining slots is correct as stated in data model. In my opinion, member_descriptor should be treated same as MISSING and everything should work correctly. |
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| msg314091 - (view) |
Author: Eric V. Smith (eric.smith) * |
Date: 2018-03-19 11:32 |
| My point is that the problem is that after: @dataclass class Base: __slots__ = ('x',) x: Any Base.x has a value (it's the member_descriptor for x). That's what's causing the problem that when adding a field to the derived class, it thinks you're adding a field without a default value after one that has a default value. I agree that I could detect this specific case and allow it. My comment about the error message was inaccurate. |
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| msg314121 - (view) |
Author: Adrian Stachlewski (stachel) |
Date: 2018-03-19 21:52 |
| There's also another major problem. Because Base.x has value, __init__ is not prepared correctly (member_descriptor is passed as default). @dataclass class Base: __slots__ = ('x',) x: Any Base() # No TypeError exception Fixing this should be quite easy, if you want I can prepare PR. |
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| msg314122 - (view) |
Author: Eric V. Smith (eric.smith) * |
Date: 2018-03-19 22:30 |
| Thanks, but I'm already looking at this in the context of a different bug. |
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| msg314124 - (view) |
Author: Eric V. Smith (eric.smith) * |
Date: 2018-03-20 01:07 |
| New changeset 7389fd935c95b4b6f094312294e703ee0de18719 by Eric V. Smith in branch 'master': bpo-33100: Dataclasses now handles __slots__ and default values correctly. (GH-6152) https://github.com/python/cpython/commit/7389fd935c95b4b6f094312294e703ee0de18719 |
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| msg314128 - (view) |
Author: Eric V. Smith (eric.smith) * |
Date: 2018-03-20 01:31 |
| New changeset 3d41f482594b6aab12a316202b3c06757262109a by Eric V. Smith (Miss Islington (bot)) in branch '3.7': bpo-33100: Dataclasses now handles __slots__ and default values correctly. (GH-6152) (GH-6153) https://github.com/python/cpython/commit/3d41f482594b6aab12a316202b3c06757262109a |
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