[Python-Dev] Concerns about method overriding and subclassing with dataclasses

[Guido van Rossum]

Hm. I don't know that people will conclude that checking for a dataclass is
an anti-pattern. They'll probably just invent a myriad of different hacks
like the one you showed. I recommend making it public.

I still worry a bit about ClassVar and InitVar being potentially useful but
I concede I have no use case so I'll drop it.

On Fri, Jan 5, 2018 at 8:43 AM, Eric V. Smith <eric at trueblade.com> wrote:

> On 1/5/2018 11:24 AM, Guido van Rossum wrote:
>
>> On Fri, Jan 5, 2018 at 5:08 AM, Eric V. Smith <eric at trueblade.com
>> <mailto:eric at trueblade.com>> wrote:
>>
>>     On 1/2/2018 12:01 AM, Guido van Rossum wrote:
>>
>>         Yes, there's a class variable (__dataclass_fields__) that
>>         identifies the parent fields. The PEP doesn't mention this or
>>         the fact that special methods (like __repr__ and __init__) can
>>         tell whether a base class is a dataclass. It probably should
>>         though. (@Eric)
>>
>>
>>     I think that's covered in this section:
>>     https://www.python.org/dev/peps/pep-0557/#inheritance
>>     <https://www.python.org/dev/peps/pep-0557/#inheritance>
>>
>>
>> I was specifically talking about the name and contents of
>> __dataclass_fields__, which are not documented by the PEP. I expect it's
>> inevitable that people will be looking at this (since they can see it in
>> the source code). Or do you recommend that people use dataclasses.fields()
>> and catch ValueError?
>>
>
> The expectation is to use dataclasses.fields(). Both it and
> __dataclass_fields__ contain the fields for this class and the parents. The
> only difference is the pseudo-fields.
>
> I can add some words describing .fields() returning which fields are
> present.
>
> I notice that _isdataclass() exists but is private and I don't recall why.
>>
>
> I think the argument was that it's an anti-pattern, and if you really want
> to know, just call dataclasses.fields() and catch the TypeError. I have
> this in a helper file:
>
> def isdataclass(obj):
>     """Returns True for dataclass classes and instances."""
>     try:
>         dataclasses.fields(obj)
>         return True
>     except TypeError:
>         return False
>
>
> (Also now I'm curious what
>
>> the "pseudo-fields" are that fields() ignores, but that's OT.)
>>
>
> ClassVar and InitVar "fields". dataclasses.fields() doesn't return them.
>
> Eric.
>
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-- 
--Guido van Rossum (python.org/~guido)
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