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`.
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