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

[Eric V. Smith]

On 1/5/2018 12:58 PM, Guido van Rossum wrote:
> 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'm trying to track down the original discussion. We got bogged down on 
whether it worked for classes or instances or both, then we got tied up 
in naming it (surprise!), then it looks like we decided to just not 
include it since you could make those decisions for yourself.

I think the discussion is buried in this thread:
https://mail.python.org/pipermail/python-dev/2017-November/150966.html

Which references:
https://github.com/ericvsmith/dataclasses/issues/99

So, ignoring the naming issue, I think if we want to revive it, the 
question is: should isdataclass() return True on just instances, just 
classes, or both? And should it ever raise an exception, or just return 
False?

> 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.

IIRC, we decided that we could add a parameter to dataclasses.fields() 
if we ever wanted to return pseudo-fields. But no one came up with a use 
case.

Eric.

> 
> On Fri, Jan 5, 2018 at 8:43 AM, Eric V. Smith <eric at trueblade.com 
> <mailto: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>
>         <mailto: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>
>              <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 <http://python.org/%7Eguido>)
> 
> 
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