Why can'y you make `name` on `NamedObjectItem` a property that returns ` self.obj.name`? Why store a duplicate copy of the name?
PS. I have to ponder why frozen dataclasses don't use `__new__`. On Fri, Feb 16, 2018 at 11:43 PM, Ben Lewis <benlew...@gmail.com> wrote: > On Sat, Feb 17, 2018 at 6:40 PM, Guido van Rossum <gu...@python.org> > wrote: >> >> >> That's a pretty tricky proposal, and one that's been debated on and off >> for a long time in other contexts. And that flag would somehow have to be >> part of every instance's state. >> > >> In general the right way to initialize an immutable/frozen object is not >> through __init__ but through __new__ -- have you tried that? >> > > Constructing it throught __new__ doesn't actually work as it has no way to > alter the arguments that are passed into __init__, I think creating a > metaclass that overides __call__ is required to acheive the desired result. > Although a factory classmethod would acheive similar api. > > >> >> Also, a small example that demonstrates your need would do wonders to >> help us understand your use case better. >> >> > > # unrelated object > class NamedObject: > @property > def name(self) -> str: > return "some name" > > // has may subclasses > @dataclass > class Item: > name: str > > > @dataclass > class NamedObjectItem(Item): > name: str = field(init=False) > obj: NamedObject > > def __post_init__(self): > self.name = self.obj.name > > This works fine, until I decided them Item and therefore all subclasses > should be frozen as no instances are mutated > and if they are ever in the future then its a bug. But to do this the > following factory method needs to be added: > > @classmethod > def create(cls, obj: NamedObject, *args, **kwargs): > return cls(obj.name, obj, *args, **kwargs) > > This doesn't look that bad but all fields(up to the last field used that > would have been used in __post_init__) needs to be declared in the > signature. > > Thanks > Ben Lewis > -- --Guido van Rossum (python.org/~guido)
_______________________________________________ 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
