On Mon, Feb 19, 2018 at 8:16 PM, Glenn Linderman <[email protected]> wrote:
> On 2/19/2018 7:02 PM, Guido van Rossum wrote: > > But how? > > On Mon, Feb 19, 2018 at 5:06 PM, Chris Barker - NOAA Federal < > [email protected]> wrote: > >> ... maybe it would be helpful to be able to >> freeze an instance after creation for multiple use-cases? >> > > And there's the crux of the issue... if the creator of Python can't figure > out how to make mutable objects immutable by fiat after creation, then it > is unlikely anyone else can! > Um, the question was meant in the Socratic way. :-) (Snipped the rest.) 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__() 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. (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.) -- --Guido van Rossum (python.org/~guido)
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