使类的常量成为其类型所在的类

debugcn 发表于 Dev

布伦特

我有一个带有特殊值“ EMPTY”和“ UNIVERSE”的Python类：

class RealSet:
    """Continuous open, half-open, and closed regions and discreet values of the Reals"""

    # implementation placeholder
    def __init__(self, intervals, *, canonicalize):
        pass

# Outside the class

RealSet.EMPTY = RealSet(tuple(), canonicalize=False)  # type: ignore
RealSet.UNIVERSE = RealSet(((None, None),), canonicalize=False)  # type: ignore

但是，棉绒，代码完成等不喜欢这种方式，因为它们不被视为类的静态属性。甚至设置它们也被报告为mypy错误，因此# type: ignore。

下面的方法不起作用，因为我不能RealSet在类范围内构造a ，因为它尚不存在：

class RealSet:
    """Continuous open, half-open, and closed regions and discreet values of the Reals"""
    ...
    ...

    EMPTY = RealSet(tuple(), canonicalize=False)  # error
    UNIVERSE = RealSet(((None, None),), canonicalize=False)  # error

而且这不起作用，因为它定义了实例属性，而不是类属性：

class RealSet:
    """Continuous open, half-open, and closed regions and discreet values of the Reals"""
    ...
    ...

    EMPTY: "RealSet"
    UNIVERSE: "RealSet"

# Outside the class

RealSet.EMPTY = RealSet(tuple(), canonicalize=False)
RealSet.UNIVERSE = RealSet(((None, None),), canonicalize=False)

这似乎是Python类设计中的一个极端案例。如何在属性的类型为其所在类的地方创建类属性？奖励：使它们恒定。

客人

您可以typing.ClassVar用来注释类变量：

class RealSet:
    def __init__(self, intervals, *, canonicalize):
        pass

    EMPTY: ClassVar['RealSet']
    UNIVERSE: ClassVar['RealSet']


RealSet.EMPTY = RealSet(tuple(), canonicalize=False)
RealSet.UNIVERSE = RealSet(((None, None),), canonicalize=False)

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