通过输入继承Python 3中的泛型类型

Question

我正在用 Python 3.6 和 mypy 进行一些输入实验。我想设计一个实体类，可以通过两种方式实例化：

• 使用普通的初始化器 (

  p = Person(name='Hannes', age=27)

  )
• 从状态对象静态 (

  p = Person.from_state(person_state)

  )。

Entity

派生自

Person

类，将状态类作为通用参数。但是，当使用 mypy 验证代码时，我收到一个错误，指出

Person.from_state

没有从它继承的类中获取状态类型：

untitled2.py:47: 错误：“Entity”的“from_state”的参数 1 具有不兼容的类型“UserState”；预期的“StateType”

我认为通过继承

Entity[UserState]

，

StateType

将绑定到

UserState

，并且子类中的方法签名将相应更新。

这是完整的代码。我已经用

?????

标记了我怀疑自己做错的地方。第 47 行几乎在底部并在代码中进行了标记。

from typing import TypeVar, Generic, NamedTuple, List, NewType

EntityId = NewType('EntityId', str)

StateType = TypeVar('StateType')

class Entity(Generic[StateType]):
    id: EntityId = None
    state: StateType = None

    @classmethod
    def from_state(cls, state: StateType): # ?????
        ret = object.__new__(cls)
        ret.id = None
        ret.state = state
        return ret

    def assign_id(self, id: EntityId) -> None:
        self.id = id

class UserState(NamedTuple):
    name: str
    age: int

class User(Entity[UserState]):
    def __init__(self, name, age) -> None:
        super().__init__()
        self.state = UserState(name=name, age=age)

    @property
    def name(self) -> str:
        return self.state.name

    @property
    def age(self) -> int:
        return self.state.age

    def have_birthday(self) -> None:
        new_age = self.state.age+1
        self.state = self.state._replace(age=new_age)

# Create first object with constructor
u1 = User(name='Anders', age=47)

# Create second object from state
user_state = UserState(name='Hannes', age=27)
u2 = User.from_state(user_state) # Line 47

print(u1.state)
print(u2.state)

Answer 1

这是 mypy 中的一个错误，在 mypy 0.700 中已被“修复”。正如评论中的一些人指出的那样，该行代码在新版本中验证得很好。 请注意，在较新版本的 mypy 中，问题中的代码存在不同的问题：

main.py:8: error: Incompatible types in assignment (expression has type "None", variable has type "EntityId")
main.py:9: error: Incompatible types in assignment (expression has type "None", variable has type "StateType")

但这超出了问题的范围，由您决定如何解决。

Answer 2

from __future__ import annotations
from typing import NamedTuple, Optional, Type


class UserState(NamedTuple):
    name: str
    age: int


class Entity:
    id: Optional[str]
    state: UserState

    @classmethod
    def from_state(cls: Type[Entity], state: UserState) -> Entity:
        entity_from_state: Entity = object.__new__(cls)
        entity_from_state.id = None
        entity_from_state.state = state
        return entity_from_state

    def assign_id(self, id: str) -> None:
        self.id = id


class User(Entity):
    def __init__(self, name: str, age: int) -> None:
        self.state = UserState(name=name, age=age)

    @property
    def name(self) -> str:
        return self.state.name

    @property
    def age(self) -> int:
        return self.state.age

    def have_birthday(self) -> None:
        new_age = self.state.age + 1
        self.state = self.state._replace(age=new_age)


# Create first object with constructor
u1 = User(name="Anders", age=47)

# Create second object from state
user_state = UserState(name="Hannes", age=27)
u2 = User.from_state(user_state)  # Line 47

print(u1.state)
print(u2.state)