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

2018年2月22日 02:58:09 -0800 

On 2/22/2018 1:56 AM, Raymond Hettinger wrote:

When working on the docs for dataclasses, something unexpected came up. If a 
dataclass is specified to be frozen, that characteristic is inherited by 
subclasses which prevents them from assigning additional attributes:
 >>> @dataclass(frozen=True)
 class D:
 x: int = 10
 >>> class S(D):
 pass
 >>> s = S()
 >>> s.cached = True
 Traceback (most recent call last):
 File "<pyshell#49>", line 1, in <module>
 s.cached = True
 File 
"/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/dataclasses.py",
 line 448, in _frozen_setattr
 raise FrozenInstanceError(f'cannot assign to field {name!r}')
 dataclasses.FrozenInstanceError: cannot assign to field 'cached'
This is because "frozen-ness" is implemented by adding __setattr__ and __delattr__ methods in D, which get inherited by S. 
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. A related issue is that dataclasses derived from frozen dataclasses are automatically "promoted" to being frozen. 
>>> @dataclass(frozen=True)
... class A:
... i: int
...
>>> @dataclass
... class B(A):
... j: int
...
>>> b = B(1, 2)
>>> b.j = 3
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
File "C:\home\eric\local\python\cpython\lib\dataclasses.py", line 452, in _frozen_setattr 
 raise FrozenInstanceError(f'cannot assign to field {name!r}')
dataclasses.FrozenInstanceError: cannot assign to field 'j'
Maybe it should be an error to declare B as non-frozen?
Eric.
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