[issue33094] dataclasses: ClassVar attributes are not working properly

[Adrian Stachlewski]

New submission from Adrian Stachlewski :

Class variables should behave in the same way whether with or without ClassVar 
annotation. Unfortunately there are not.

class A:
__slots__ = ()
x: ClassVar = set()

A()  # it's ok

@dataclass
class B:
__slots__ = ()
x = set()

B()  # ok too

@dataclass
class C:
__slots__ = ()
# cannot use set() because of error
x: ClassVar = field(default_factory=set) 

C()  # AttributeError: 'C' object has no attribute 'x'

Exception is raised from __init__ method, with flag init=False nothing changes. 

Python version: 3.7.0b2

--
components: Library (Lib)
messages: 314017
nosy: stachel
priority: normal
severity: normal
status: open
title: dataclasses: ClassVar attributes are not working properly
type: behavior
versions: Python 3.7

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[issue33094] dataclasses: ClassVar attributes are not working properly

[Adrian Stachlewski]

Adrian Stachlewski  added the comment:

Thanks for explaining. I was trying to do something like

@dataclass
class A:
  x: ClassVar = set()

and thanks to you I know it should be 

@dataclass
class A:
  x: ClassVar[Set] = set()

If you are looking for improved error message, it's probably should be somehow 
connected to not proper usage of annotation. I've ended with default_factory 
because x: ClassVar = set() wasn't working and there was no error with 
default_factory and without slots.

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[issue33100] dataclasses and __slots__ - non-default argument (member_descriptor)

[Adrian Stachlewski]

New submission from Adrian Stachlewski :

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.

--
components: Library (Lib)
messages: 314077
nosy: stachel
priority: normal
severity: normal
status: open
title: dataclasses and __slots__ - non-default argument (member_descriptor)
type: behavior
versions: Python 3.7, Python 3.8

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[issue33094] dataclasses: ClassVar attributes are not working properly

[Adrian Stachlewski]

Adrian Stachlewski  added the comment:

Once more same mistake. 

'x' should be declared as: 
- x: ClassVar[set] = set()
- x: ClassVar[Set[Any]] = set()

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[issue33100] dataclasses and __slots__ - non-default argument (member_descriptor)

[Adrian Stachlewski]

Adrian Stachlewski  added the comment:

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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[issue33100] dataclasses and __slots__ - non-default argument (member_descriptor)

[Adrian Stachlewski]

Adrian Stachlewski  added the comment:

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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[issue33094] dataclasses: ClassVar attributes are not working properly

[Adrian Stachlewski]

Adrian Stachlewski  added the comment:

There's nothing to do, thanks for help one more again.

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status: pending -> open

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