在数据 Class 中创建 class 变量的正确方法

Question

我刚刚开始使用 Python 的数据 Classes，我想确认我是否以正确的方式声明了 Class 变量。

使用常规 python classes

class Employee:

    raise_amount = .05

    def __init__(self, fname, lname, pay):
        self.fname = fname
        self.lname = lname
        self.pay = pay

使用 python 数据 Class

@dataclass
class Employee:
    fname: str
    lname: str
    pay: int
    raise_amount = .05

我指的class变量是raise_amount。这是使用 Data Classes 正确声明的 class 变量吗？或者有更好的方法吗？

我已经测试了数据 class 实现，它提供了预期的功能，但我主要想知道我的实现是否遵循了最佳实践。

Answer 1

要创建 class 变量，请将字段注释为 typing.ClassVar 或根本不注释。

from typing import ClassVar

@dataclass
class Foo:
    ivar: float = 0.5
    cvar: ClassVar[float] = 0.5
    nvar = 0.5

foo = Foo()
Foo.ivar, Foo.cvar, Foo.nvar = 1, 1, 1
print(Foo().ivar, Foo().cvar, Foo().nvar)   # 0.5 1 1
print(foo.ivar, foo.cvar, foo.nvar)         # 0.5 1 1

有一个细微的区别，未注释的字段被 @dataclass 完全忽略，而 ClassVar 字段被存储但未转换为属性。

---

dataclasses — Data Classes

The member variables [...] are defined using PEP 526 type annotations.

Class variables

One of two places where dataclass() actually inspects the type of a field is to determine if a field is a class variable as defined in PEP 526. It does this by checking if the type of the field is typing.ClassVar. If a field is a ClassVar, it is excluded from consideration as a field and is ignored by the dataclass mechanisms. Such ClassVar pseudo-fields are not returned by the module-level fields() function.