[Python-Dev] Second post: PEP 557, Data Classes

[Eric V. Smith]

On 11/27/2017 10:51 AM, Guido van Rossum wrote:
> Following up on this subthread (inline below).

> Didn't we at one point have something like
> 
> isinstance(other, self.__class__) and fields(other) == fields(self) and 
> <all individual fields match>
> 
> (plus some optimization if the types are identical)?
> 
> That feels ideal, because it means you can subclass Point just to add 
> some methods and it will stay comparable, but if you add fields it will 
> always be unequal.

One thing this doesn't let you do is compare instances of two different 
subclasses of a base type:

@dataclass
class B:
     i: int

@dataclass
class C1(B): pass

@dataclass
class C2(B): pass

You can't compare C1(0) and C2(0), because neither one is an instance of 
the other's type. The test to get this case to work would be expensive: 
find the common ancestor, and then make sure no fields have been added 
since then. And I haven't thought through multiple inheritance.

I suggest we don't try to support this case.

Eric.