如何在 Python 中声明未初始化变量的类型？ [复制]

Question

在练习算法和数据结构的同时，我正在学习使用 mypy 和静态类型检查器。

在二叉搜索树中，节点被初始化为没有子节点。它们是节点类型。但是，似乎 None 在 Python 中是它自己的对象类型，所以 mypy 在下面给了我一个错误。是否可以将未分配的孩子初始化为类型节点？

binary_search_tree.py:17: error: Incompatible types in assignment (expression has type "None", variable has type "Node")
binary_search_tree.py:18: error: Incompatible types in assignment (expression has type "None", variable has type "Node")
Found 2 errors in 1 file (checked 1 source file)

代码如下：

class Node:
    # A node has a value which is an int and two children which are nodes
    def __init__(self, value: int):
        self.value: int = value
        self.left: Node = None
        self.right: Node = None 

Answer 1

因为这些值有时可能是None，所以您应该将它们指定为Optional 类型，然后在使用它们时执行显式None 检查，以便mypy 知道它们有一个值。更多信息来自mypy的文档here：

from typing import Optional

class Node:
    # A node has a value which is an int and two children which are nodes
    def __init__(self, value: int):
        self.value: int = value
        self.left: Optional[Node] = None
        self.right: Optional[Node] = None

如果将它们初始化为None，则不可能仅将它们声明为Node 类型。另一种选择是创建一个NullNode（或类似的）子类，它仍然具有Node 类型，但表示那里没有Node：

class Node:
    # A node has a value which is an int and two children which are nodes
    def __init__(self, value: int):
        self.value: int = value
        self.left: Node = NullNode()
        self.right: Node = NullNode()

class NullNode(Node):
    pass