On Tue, Feb 20, 2018 at 8:45 AM, Guido van Rossum <gu...@python.org> wrote:
> TBH, I don't hate Nick's solution that assigns to __class__ in > __post_init__. You can probably come up with a class decorator that hides > construction of the frozen subclass. I do think that it should be used very > sparingly, as the existence of the subclass is hard to hide: instances of > MyRecord will show as instances of _LockedMyRecord when printed or > otherwise inspected, which could be confusing. > > There's another very simple low-tech solution: > > @dataclass > class MyRecord: > a: int > # etc. > frozen: bool = False > def freeze(self): > self.frozen = True > def __setattr__(self, name, value): > if self.frozen: raise AttributeError > super().__setattr__(name, value) > def __hash__(self): > assert self.frozen > return super().__hash__() > This sure seems far more straightforward and les magic than __class__ swapping. > It can easily be weaponized via an extra decorator, and it's simpler in > that it doesn't use a magical second class. I'm just not keen on adding the > extra attribute to all frozen instances created via > @dataclass(frozen=True), because it's extra space overhead. > Is that one attribute that big a deal? I suppose that simple dataclasses with only two or so attributes would see a significant change, but if you're really worried about space, wouldn't you use a namedtuple or simply a tuple anyway? (In fact you can conclude that I'm not keen on exposing *any* of these > "solutions" as a flag on the @dataclass() decorator. They all feel pretty > fragile to me.) > so provide a: @freezable_dataclass Additional decorator that folks that want the ability to freeze and unfreeze instances can use? and if it overrides __set__attr, it could be used an virtually any class to make a frozen version, yes? I can't say I have enough need to bother writing that myself, but if others do -- it seems like a good solution. -CHB -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception chris.bar...@noaa.gov
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