On 2/4/2018 9:49 PM, Guido van Rossum wrote:
A frozen class requires a lot of discipline, since you have to compute the values of all fields before calling the constructor. A mutable class allows other initialization patterns, e.g. manually setting some fields after the instance has been constructed, or having a separate non-dunder init() method. There may be good reasons for using these patterns, e.g. the object may be part of a cycle (e.g. parent/child links in a tree). Or you may just use one of these patterns because you're a pretty casual coder. Or you're modeling something external.


My point is that once you have one of those patterns in place, changing your code to avoid them may be difficult. And yet your code may treat the objects as essentially immutable after the initialization phase (e.g. a parse tree). So if you create a dataclass and start coding like that for a while, and much later you need to put one of these into a set or use it as a dict key, switching to frozen=True may not be a quick option. And writing a __hash__ method by hand may feel like a lot of busywork. So this is where [unsafe_]hash=True would come in handy.

I think naming the flag unsafe_hash should take away most objections, since it will be clear that this is not a safe thing to do. People who don't understand the danger are likely to copy a worse solution from StackOverflow anyway. The docs can point to frozen=True and explain the danger.

This is an interesting use case. I haven't got the internals knowledge to know just how just different mutable and immutable classes and objects are under the hood. But this use case makes me wonder if, even at the cost of some performance that "normal" immutable classes and objects might obtain, if it would be possible to use the various undisciplined initialization patterns as desired, followed by as declaration "This OBJECT is now immutable" which would calculate its HASH value, and prevent future mutations of the object?

Yes, I'm aware that the decision for immutability has historically been done at the class level, not the object level, but in my ignorance of the internals, I wonder if that is necessary, for performance or more importantly, for other reasons.

And perhaps the implementation is internally almost like two classes, one mutable, and the other immutable, and the declaration would convert the object from one to the other.  But if I say more, I'd just be babbling.
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
https://mail.python.org/mailman/listinfo/python-dev
Unsubscribe: 
https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com

Reply via email to