[Python-Dev] Should the dataclass frozen property apply to subclasses?

[Nick Coghlan]

On 28 February 2018 at 10:37, Eric V. Smith <eric at trueblade.com> wrote:

> So, given a frozen dataclass "C" with field names in "field_names", I
> propose changing __setattr__ to be:
>
> def __setattr__(self, name, value):
>     if type(self) is C or name in field_names:
>         raise FrozenInstanceError(f'cannot assign to field {name!r}')
>     super(cls, self).__setattr__(name, value)
>
> In the current 3.7.0a2 implementation of frozen dataclasses, __setattr__
> always raises. The change is the test and then call to super().__setattr__
> if it's a derived class. The result is an exception if either self is an
> instance of C, or if self is an instance of a derived class, but the
> attribute being set is a field of C.
>

I'm assuming you meant "3.7.0b2" here (and similarly alpha->beta for the
other version numbers below)


> So going back to original questions above, my suggestions are:
>
> 1. What happens when a frozen dataclass inherits from a non-frozen
> dataclass? The result is a frozen dataclass, and all fields are
> non-writable. No non-fields can be added. This is a reversion to the
> 3.7.0a1 behavior.
>
> 2. What happens when a non-frozen dataclass inherits from a frozen
> dataclass? The result is a frozen dataclass, and all fields are
> non-writable. No non-fields can be added. This is a reversion to the
> 3.7.0a1 behavior. I'd also be okay with this case being an error, and you'd
> have to explicitly mark the derived class as frozen. This is the 3.7.0a2
> behavior.
>
> 3. What happens when a non-dataclass inherits from a frozen dataclass? The
> fields that are in the dataclass are non-writable, but new non-field
> attributes can be added. This is new behavior.
>
> 4. Can new non-field attributes be created for frozen dataclasses? No.
> This is existing behavior.
>

+1 from me for the variant of this where dataclasses inheriting from frozen
data classes must explicitly mark themselves as frozen (at least for now).
That way we leave the door open to allowing a variant where a non-frozen
dataclass that inherits from a frozen dataclass can set "hash=False" on all
of the new fields it adds to avoid becoming frozen itself.


> I'm hoping this change isn't so large that we can't get it in to 3.7.0a3
> next month.
>

I think this qualifies as the beta period serving its intended purpose
(i.e. reviewing and refining the behaviour of already added features,
without allowing completely new features).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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