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

Ivan Levkivskyi levkivskyi at gmail.com
Thu Feb 22 06:04:49 EST 2018 
On 22 February 2018 at 10:55, Eric V. Smith <eric at trueblade.com> wrote:

> On 2/22/2018 1:56 AM, Raymond Hettinger wrote:
>
>> Other immutable classes in Python don't behave the same way:
>
>
>>      >>> class T(tuple):
>>              pass
>>
>>      >>> t = T([10, 20, 30])
>>      >>> t.cached = True
>>
>>      >>> class F(frozenset):
>>              pass
>>
>>      >>> f = F([10, 20, 30])
>>      >>> f.cached = True
>>
>>      >>> class B(bytes):
>>              pass
>>
>>      >>> b = B()
>>      >>> b.cached = True
>>
>
> The only way I can think of emulating this is checking in __setattr__ to
> see if the field name is a field of the frozen class, and only raising an
> error in that case.
>

How about checking that the type of self is the type where decorator was
applied? For example (pseudocode):

def dataclass(cls, ...):
    def _set_attr(self, attr, value):
        if type(self) is not cls:
             use super()
        else:
            raise AttributeError
    cls.__setattr__ = _set_attr

It can be also more sophisticated, for example raising for all fields on
class where frozen=True was used, while only on frozen fields for
subclasses.

--
Ivan
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