[Python-Dev] Is static typing still optional?

[Raymond Hettinger]

>>> 2) Change the default value for "hash" from "None" to "False".  This might take a little effort because there is currently an oddity where setting hash=False causes it to be hashable.  I'm pretty sure this wasn't intended ;-)
>> I haven't looked at this yet.
> 
> I think the hashing logic explained in https://bugs.python.org/issue32513#msg310830 is correct. It uses hash=None as the default, so that frozen=True objects are hashable, which they would not be if hash=False were the default.

Wouldn't it be simpler to make the options orthogonal?  Frozen need not imply hashable.  I would think if a user wants frozen and hashable, they could just write frozen=True and hashable=True.  That would more explicit and clear than just having frozen=True imply that hashability gets turned-on implicitly whether you want it or not.

> If there's some case there that you disagree with, I'd be interested in hearing about it.
> 
> That logic is what is currently scheduled to go in to 3.7 beta 1. I have not updated the PEP yet, mostly because it's so difficult to explain.

That might be a strong hint that this part of the API needs to be simplified :-)

"If the implementation is hard to explain, it's a bad idea." -- Zen

If for some reason, dataclasses really do need tri-state logic, it may be better off with enum values (NOT_HASHABLE, VALUE_HASHABLE, IDENTITY_HASHABLE, HASHABLE_IF_FROZEN or some such) rather than with None, True, and False which don't communicate enough information to understand what the decorator is doing.

> What's the case where setting hash=False causes it to be hashable? I don't think that was ever the case, and I hope it's not the case now.

Python 3.7.0a4+ (heads/master:631fd38dbf, Jan 28 2018, 16:20:11) 
[GCC 7.2.0] on darwin
Type "copyright", "credits" or "license()" for more information.

>>> from dataclasses import dataclass
>>> @dataclass(hash=False)
class A:
    x: int

>>> hash(A(1))
285969507


I'm hoping that this part of the API gets thought through before it gets set in stone.  Since dataclasses code never got a chance to live in the wild (on PyPI or some such), it behooves us to think through all the usability issues.  To me at least, the tri-state hashability was entirely unexpected and hard to debug -- I had to do a close reading of the source to figure-out what was happening.


Raymond