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'
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
Raymond
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