英文:
TypeError when using super() in a dataclass with slots=True
问题
这段代码中，当你在使用 dataclass 的时候，添加了 slots=True，就会导致 super() 的调用出现错误。为了修复这个问题，你可以在 super() 中使用类名而不是 self，像这样：
data = super(B, self).get_data()
这会正确地调用父类的方法，而不会引发 TypeError。这是因为 slots=True 会创建一个新的类来代替原始的类，所以需要显式指定类名。
英文:
I have a dataclass with (kind of) a getter method.
This code works as expected:
from dataclasses import dataclass@dataclass()class A:def get_data(self):# get some values from object's fields# do some calculationsreturn "a calculated value"@dataclass()class B(A):def get_data(self):data = super().get_data()return data + " (modified)"b = B()print(b.get_data()) # a calculated value (modified)
However, if I add slots=True, I get a TypeError:
from dataclasses import dataclass@dataclass(slots=True)class A:def get_data(self):return "a calculated value"@dataclass(slots=True)class B(A):def get_data(self):data = super().get_data()return data + " (modified)"b = B()print(b.get_data()) # TypeError: super(type, obj): obj must be an instance or subtype of type
The error vanishes if I use the old-style super(), contrary to pep-3135:
from dataclasses import dataclass@dataclass(slots=True)class A:def get_data(self):return "a calculated value"@dataclass(slots=True)class B(A):def get_data(self):data = super(B, self).get_data()return data + " (modified)"b = B()print(b.get_data()) # a calculated value (modified)
Why does this happen and how to fix it the right way?
答案1
得分: 2
Sure, here's the translated content:
slots：如果设置为True（默认为False），将生成__slots__属性，并返回一个新的类，而不是原始类。如果类中已经定义了__slots__，则会引发TypeError。
以此为例：
@dataclass(slots=True)class Foo:pass
这意味着它的工作方式类似于：
class Foo:pass
Foo = dataclass(slots=True)(Foo)
你定义了一个名为
Foo的类，然后它被替换为一个不同的，被修改过的类。
现在，你的方法：
def get_data(self):data = super().get_data()...
这个
super()是在原始的Foo类中编写的，它会假设它应该查找这个原始Foo类的父类；但它当前具有的实例实际上不是该类的实例，而是另一个经过修改的类的实例。
当你改用这种方式：
data = super(Foo, self).get_data()
这会查找当前引用名称
Foo的内容，这与此时self引用的内容相匹配。
英文:
> slots: If true (the default is False), __slots__ attribute will be generated and new class will be returned instead of the original one. If __slots__ is already defined in the class, then TypeError is raised.
>
> https://docs.python.org/3/library/dataclasses.html#dataclasses.dataclass
Taking this as an example:
@dataclass(slots=True)class Foo:pass
This means this works something like this:
class Foo:passFoo = dataclass(slots=True)(Foo)
You define a class Foo, and then it gets replaced with a different, altered class.
Now, your method:
def get_data(self):data = super().get_data()...
This super() was written in the original Foo class and will assume it's supposed to look up the parent of this original Foo class; but the instance it currently has is not actually an instance of that class, it's an instance of the other, altered class.
When you do this instead:
data = super(Foo, self).get_data()
This looks up what currently refers to the name Foo, which again matches what self also refers to at this moment.
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