[Python-Dev] Issues with PEP 526 Variable Notation at the class level

[Guido van Rossum]

Yes, I think this is a reasonable argument for adding a 'slots' option (off
by default) for @dataclass(). However I don't think we need to rush it in.
I'm not very happy with the general idea of slots any more, and I think
that it's probably being overused, and at the same time I expect that there
are a lot of classes with a slots declaration that still have a dict as
well, because they inherit from a class without slots.

I'm not sure what to do about docstrings -- I'm not a big user of pydoc and
I find help() often too verbose (I usually read the source. Maybe we could
add a 'doc' option to field()? That's similar to what we offer for
property().

On Thu, Dec 7, 2017 at 12:47 PM, Eric V. Smith <eric at trueblade.com> wrote:

> On 12/7/17 3:27 PM, Raymond Hettinger wrote:
> ...
>
> I'm looking for guidance or workarounds for two issues that have arisen.
>>
>> First, the use of default values seems to completely preclude the use of
>> __slots__.  For example, this raises a ValueError:
>>
>>     class A:
>>         __slots__ = ['x', 'y']
>>         x: int = 10
>>         y: int = 20
>>
>
> Hmm, I wasn't aware of that. I'm not sure I understand why that's an
> error. Maybe it could be fixed?
>
> Otherwise, I have a decorator that takes a dataclass and returns a new
> class with slots set:
>
> >>> from dataclasses import dataclass
> >>> from dataclass_tools import add_slots
> >>> @add_slots
> ... @dataclass
> ... class C:
> ...   x: int = 0
> ...   y: int = 0
> ...
> >>> c = C()
> >>> c
> C(x=0, y=0)
> >>> c.z = 3
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> AttributeError: 'C' object has no attribute 'z'
>
> This doesn't help the general case (your class A), but it does at least
> solve it for dataclasses. Whether it should be actually included, and what
> the interface would look like, can be (and I'm sure will be!) argued.
>
> The reason I didn't include it (as @dataclass(slots=True)) is because it
> has to return a new class, and the rest of the dataclass features just
> modifies the given class in place. I wanted to maintain that conceptual
> simplicity. But this might be a reason to abandon that. For what it's
> worth, attrs does have an @attr.s(slots=True) that returns a new class with
> __slots__ set.
>
> The second issue is that the different annotations give different
>> signatures than would produced for manually written classes.  It is unclear
>> what the best practice is for where to put the annotations and their
>> associated docstrings.
>>
>
> I don't have any suggestions here.
>
> Eric.
>
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-- 
--Guido van Rossum (python.org/~guido)
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