On Sat, Feb 17, 2018 at 6:40 PM, Guido van Rossum <[email protected]> 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
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