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On Tue, Nov 28, 2017 at 12:56 PM, Eric V. Smith <eric@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 bb 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.