返回 self 的 python 通用屬性用法的返回值類型不兼容

Question

嘗試實現尊重mypy的節點和樹 inheritance 層次結構，我有下一個類：

from __future__ import annotations
import dataclasses
from typing import Generic, TypeVar, Optional

GenericNodeValue = TypeVar('GenericNodeValue')

@dataclasses.dataclass
class BaseNode(Generic[GenericNodeValue]):
    value: GenericNodeValue
    parent: Optional[BaseNode] = None
    left: Optional[BaseNode] = None
    right: Optional[BaseNode] = None

    @property
    def dummy(self) -> BaseNode:
        return self

GenericNode = TypeVar('GenericNode', bound=BaseNode)

class BaseTree(Generic[GenericNode]):
    def __init__(self):
        self.root: Optional[GenericNode] = None

    def get_dummy(self, node: GenericNode) -> GenericNode:
        return node.dummy

@dataclasses.dataclass
class RedBlackTreeNode(BaseNode[int]):
    red: bool = False

class RedBlackTree(BaseTree[RedBlackTreeNode]):
    def get_dummy2(self, node: RedBlackTreeNode) -> RedBlackTreeNode:
        return node.dummy

但mypy返回錯誤：

test.py:29: error: Incompatible return value type (got "BaseNode[Any]", expected "GenericNode")
test.py:39: error: Incompatible return value type (got "BaseNode[Any]", expected "RedBlackTreeNode")

有什么方法可以使用 python 輸入來實現我的邏輯嗎？

Answer 1

訣竅是向self參數添加注釋。 例如：

T = TypeVar('T')
V = TypeVar('V', bound='BaseNode')

@dataclasses.dataclass
class BaseNode(Generic[T]):
    value: T
    parent: Optional[BaseNode[T]] = None
    left: Optional[BaseNode[T]] = None
    right: Optional[BaseNode[T]] = None

    @property
    def dummy(self: V) -> V:
        return self

    @property
    def dummy_parent(self: V) -> V:
        assert isinstance(self.parent, type(self))
        return self.parent

順便說一句，您應該將類型參數添加到可選屬性中，這樣它們就不會變成BaseNode[Any] ，失去泛型的好處！