[英]Python 3 base class type annotation only allow the current subclass
Let's say I've got three classes, one parent class, and two subclasses:假设我有三个类,一个父类和两个子类:
class BaseModel:
def merge(self, other):
return self + other
class ChildA(BaseModel):
pass
class ChildB(BaseModel):
pass
The parent class has a method that takes another instance of the current class and returns a new instance of the current class (out of scope for this question).父类有一个方法,它接受当前类的另一个实例并返回当前类的一个新实例(超出了这个问题的范围)。
How do I annotate BaseModel.merge
to restrict it to only the current subclass?如何注释
BaseModel.merge
以将其限制为仅当前子类?
I can do something like this:我可以做这样的事情:
def merge(self, other: BaseModel) -> BaseModel:
return self + other
But this still allows me to pass an instance of ChildB
into ChildA
, since both inherit from BaseModel
.但这仍然允许我将
ChildA
的实例ChildB
给ChildA
,因为两者都继承自BaseModel
。 I only want ChildA
to be allowed in ChildA
, and ChildB
to be allowed for ChildB
.我只想
ChildA
在被允许ChildA
,并ChildB
被允许ChildB
。 How can I do that without reimplementing merge
on each subclass?如何在不重新实现每个子类的
merge
情况下做到这一点?
Annotate both arguments with a type variable, to enforce that both arguments must be of the same type.用类型变量注释两个参数,以强制两个参数必须是相同的类型。
from typing import TypeVar
B = TypeVar('B', bound='BaseModel')
class BaseModel:
def __init__(self, x: int):
self.x = x
def __add__(self: B, other: B) -> B:
return type(self)(self.x + other.x)
def merge(self: B, other: B) -> B:
return self + other
class ChildA(BaseModel):
pass
class ChildB(BaseModel):
pass
print(ChildA(3).merge(ChildA(4)).x) # Valid; both arguments are ChildA
print(ChildA(3).merge(ChildB(4)).x) # Invalid; one ChildA and one ChildB
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