问题描述
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
然而,linting、代码完成等不喜欢这样,因为它们不被视为类的静态属性。即使设置它们也会报告为 mypy 错误,因此 # type: ignore
.
以下不起作用,因为我无法在类范围内构造 RealSet
,因为它尚不存在:
class RealSet:
"""Continuous open,and closed regions and discreet values of the Reals"""
...
...
EMPTY = RealSet(tuple(),canonicalize=False) # error
UNIVERSE = RealSet(((None,canonicalize=False) # error
class RealSet:
"""Continuous 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,canonicalize=False)
这似乎是 Python 类设计中的一个极端情况。如何创建类属性,其中属性的类型是它所在的类?奖励:使它们保持不变。
解决方法
您可以使用 typing.ClassVar
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)
,
从 Python 3.9 开始 classmethod
可以修饰其他描述符,例如 property
。这样就可以创建一个“类属性”:
class RealSet:
def __init__(self,canonicalize):
pass
@classmethod
@property
def EMPTY(cls):
return cls(tuple(),canonicalize=False)
@classmethod
@property
def UNIVERSE(cls):
return cls(((None,canonicalize=False)