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[Python-Dev] Issues with PEP 526 Variable Notation at the class level

Nathaniel Smith njs at pobox.com
Fri Dec 8 21:14:58 EST 2017 
On Dec 7, 2017 12:49, "Eric V. Smith" <eric at trueblade.com> wrote:

The reason I didn't include it (as @dataclass(slots=True)) is because it
has to return a new class, and the rest of the dataclass features just
modifies the given class in place. I wanted to maintain that conceptual
simplicity. But this might be a reason to abandon that. For what it's
worth, attrs does have an @attr.s(slots=True) that returns a new class with
__slots__ set.


They actually switched to always returning a new class, regardless of
whether slots is set:

https://github.com/python-attrs/attrs/pull/260

You'd have to ask Hynek to get the full rationale, but I believe it was
both for consistency with slot classes, and for consistency with regular
class definition. For example, type.__new__ actually does different things
depending on whether it sees an __eq__ method, so adding a method after the
fact led to weird bugs with hashing. That class of bug goes away if you
always set up the autogenerated methods and then call type.__new__.

 -n
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