[Python-Dev] Is static typing still optional? (original) (raw)

Raymond Hettinger raymond.hettinger at gmail.com
Sun Jan 28 20:07:02 EST 2018

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

More information about the Python-Dev mailing list