python - 如何使数据类中的属性只读？

Question

假设我有一个这样的类(class):

class C:

    def __init__(self, stuff: int):
        self._stuff = stuff

    @property
    def stuff(self) -> int:
        return self._stuff

然后 stuff是只读的:

c = C(stuff=10)
print(c.stuff)  # prints 10

和

c.stuff = 2

按预期失败

AttributeError: can't set attribute

如何使用数据类获得相同的行为？如果我也想拥有一个 setter ， 我可以:

@dataclass
class DC:
    stuff: int
    _stuff: int = field(init=False, repr=False)

    @property
    def stuff(self) -> int:
        return self._stuff

    @stuff.setter
    def stuff(self, stuff: int):
        self._stuff = stuff

但是如果没有 @stuff.setter 我怎么能做到呢？部分？

Answer 1

获得样板减少dataclass我发现唯一的方法是使用描述符。

In [236]: from dataclasses import dataclass, field
In [237]: class SetOnce:
     ...:     def __init__(self):
     ...:         self.block_set = False
     ...:     def __set_name__(self, owner, attr):
     ...:         self.owner = owner.__name__
     ...:         self.attr = attr
     ...:     def __get__(self, instance, owner):
     ...:         return getattr(instance, f"_{self.attr}")
     ...:     def __set__(self, instance, value):
     ...:         if not self.block_set:
     ...:             self.block_set = True
     ...:             setattr(instance, f"_{self.attr}", value)
     ...:         else:
     ...:             raise AttributeError(f"{self.owner}.{self.attr} cannot be set.")

In [239]: @dataclass
     ...: class Foo:
     ...:     bar:str = field(default=SetOnce())

In [240]: test = Foo("bar")

In [241]: test.bar
Out[241]: 'bar'

In [242]: test.bar = 1
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-242-9cc7975cd08b> in <module>
----> 1 test.bar = 1

<ipython-input-237-bddce9441c9a> in __set__(self, instance, value)
     12             self.value = value
     13         else:
---> 14             raise AttributeError(f"{self.owner}.{self.attr} cannot be set.")
     15

AttributeError: Foo.bar cannot be set.

In [243]: test
Out[247]: Foo(bar='bar')