How to declare types for uninitialized variables in Python?

I'm learning to use mypy and static type checkers while practicing algorithms and data structures.

In a Binary Search Tree, a Node is initialized with no children. They are of type Node. However, it seems that None is its own object type in Python so mypy gives me an error below. Is it possible to initialize the unassigned children as type Node?

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)

Code below:

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 

Because these values can be None at times, you should instead specify them as Optional types and then perform explicit None checks when using them so that mypy will know that they have a value. More information from mypy's docs 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

It is otherwise impossible to declare them as only a Node type if you initialize them as None. Another option would be to create a NullNode (or something similar) subclass which still has the type Node but signifies that there is no Node there:

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