On 22 February 2018 at 10:55, Eric V. Smith <e...@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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