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

[Nick Coghlan]

On 28 November 2017 at 17:41, Eric V. Smith <eric at trueblade.com> wrote:
> 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.

That gets you onto problematic ground as far as transitivity is
concerned, since you'd end up with the following:

    >>> b = B(0); c1 = C1(0); c2 = C2(0)
    >>> c1 == b
    True
    >>> b == c2
    True
    >>> c1 == c2
    False

However, I think you can fix this by injecting the first base in the
MRO that defines a data field as a "__field_layout__" class attribute,
and then have the comparison methods check for "other.__field_layout__
is self.__field_layout__", rather than checking the runtime class
directly.

So in the above example, you would have:

   >>> B.__field_layout__ is B
   True
   >>> C1.__field_layout__ is B
   True
   >>> C2.__field_layout__ is B
   True

It would then be up to the dataclass decorator to set
`__field_layout__` correctly, using the follow rules:

1. Use the just-defined class if the class defines any fields
2. Use the just-defined class if it inherits from multiple base
classes that define fields and don't already share an MRO
3. Use a base class if that's either the only base class that defines
fields, or if all other base classes that define fields are already in
the MRO of that base class

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia