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

Guido van Rossum guido at python.org
Tue Nov 28 16:14:35 EST 2017

On Tue, Nov 28, 2017 at 12:56 PM, Eric V. Smith <eric at trueblade.com> wrote:

> On 11/28/2017 1:57 PM, Guido van Rossum wrote:
>> I would also be happy with a retreat, where we define `__eq__` to insist
>> that the classes are the same, and people can override this to their
>> hearts' content.
> I agree. And I guess we could always add it later, if there's a huge
> demand and someone writes a decent implementation. There would be a slight
> backwards incompatibility, though. Frankly, I think it would never be
> needed.
> One question remains: do we do what attrs does: for `__eq__` and `__ne__`
> use an exact type match, and for the 4 ordered comparison operators use an
> isinstance check? On the comparison operators, they also ignore attributes
> defined on any derived class [0]. As I said, I couldn't find an attrs issue
> that discusses their choice. I'll ask Hynek over on the dataclasses github
> issue.
> Currently the dataclasses code on master uses an exact type match for all
> 6 methods.
> Eric.
> [0] That is, they do the following (using dataclasses terms):
> Given:
> @dataclass
> class B:
>     i: int
>     j: int
> @dataclass
> class C:
>     k: int
> Then B.__eq__ is:
> def __eq__(self, other):
>     if other.__class__ is self.__class__:
>         return (other.i, other.j) == (self.i, self.j)
>     return NotImplemented
> And B.__lt__ is:
> def __lt__(self, other):
>     if isinstance(other, self.__class__):
>         return (other.i, other.j) < (self.i, self.j)
>     return NotImplemented
> So if you do:
> b = B(1, 2)
> c = C(1, 2, 3)
> Then `B(1, 2) < C(1, 2, 3)` ignores `c.k`.

Hm. Maybe for the ordering comparisons we could defer to the class with the
longest list of fields, as long as there's a subtype relationship? That way
b<c and c>b would be equivalent, and both would use C.__gt__. Which had
better not reject this on the basis that other is not an instance of a
subclass of C.

IIRC there's already something in the interpreter that tries the most
derived class first for binary operators -- that may force our hand here.

--Guido van Rossum (python.org/~guido)
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