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

[Eric V. Smith]

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.