英文:
How to Type Hint a variable that must be an instance of a class or of its subclasses in Python?
问题
Sure, here's the translated content:
让我们假设我有一个名为Animal的类以及Dog和Cat等Animal的子类。
我想要类型提示一个名为a的变量，以声明它既可以是Animal的实例，也可以是Animal实例的子类之一。
- 我尝试进行类型提示的示例：
class Animal:
pass
class Dog(Animal):
pass
class Cat(Animal):
pass
a: type[Animal] = Dog() #但也可以是Animal()或Cat()
如您所见，我一直在使用这种语法：type[ParentClassName]。
但是，根据我在类型提示文档中阅读的内容，type[ParentClassName] 用于类，而不是类的实例（参见：https://docs.python.org/3/library/typing.html#typing.Type）。
（请注意，type[ClassName]与Python 3.9以后的版本中的typing.Type[ClassName]是相同的）
我正在寻找类型提示的主要重点是它既可以是Animal的实例，也可以是Dog/Cat的实例，同时也符合mypy的要求。
- 所以是否有一种方法可以通过类型提示来实现这一点？
对于任何错误，对不起，英语不是我的母语。
感谢任何尝试回答我的问题的人。
英文:
Let's say that I have a class Animal and some subclasses of Animal like Dog and Cat.
I want to type hint a variable named a in order to declare that it can be both an Animal's instance or one of the subclass of Animal instance.
- Example of me trying to type hint :
class Animal :
pass
class Dog(Animal) :
pass
class Cat(Animal) :
pass
a : type[Animal] = Dog() #But could have been Animal() or Cat()
As you can see I have been using this syntax : type[ParentClassName] .
However, as I have read on the typing documentation, type[ParentClassName] is for a class and not instances of a class (see : https://docs.python.org/3/library/typing.html#typing.Type).
(Note that type[ClassName] is the same as typing.Type[ClassName] since Python 3.9)
The main focus of the type hint I'm searching for is the fact that it can be both an Animal's instance or a Dog/Cat's instance while being mypy friendly.
- So is there a way to do this by type hinting ?
Sorry for any mistakes, english isn't my first language.
And thanks to anyone who will try to answer my question.
答案1
得分: 0
首先，我的目标是声明变量 a 是一个 Animal。
但我担心的是，如果 a 成为 Dog 的一个实例，即使 Dog 继承自 Animal，a 仍然会被视为一个 Animal。
答案是肯定的，所以使用类型提示来声明的方式是：
class Animal:
pass
class Dog(Animal):
pass
a: Animal = Dog()
实际上，这种正确性来自于在 PEP 483 中描述的 子类型关系 的概念（请参阅：https://peps.python.org/pep-0483/#subtype-relationships）。
感谢 Brian61354270 和 juanpa.arrivillaga 提供的答案。
英文:
Firstly my goal was to have a way to declare that my variable a is an Animal.
However my concern was that if a become an instance of Dog , even if Dog inherit from Animal, would a still be considered an Animal.
The answer is yes and so the way to declare it with type hint is :
class Animal :
pass
class Dog(Animal) :
pass
a: Animal = Dog()
In fact the reason why it's correct come from the Subtype relationships notion described in PEP 483 (see : https://peps.python.org/pep-0483/#subtype-relationships).
Thanks to Brian61354270 and juanpa.arrivillaga for their answers.
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