Referencing this custom class with typing

Question

I want to define a Node for a Linked List, using type hints. An instance of Node needs a reference to another instance of Node, so I want to accept an optional parameter of type Node in my __init__.

I tried this:

from typing import Optional

class Node:
    def __init__(self, value: int, next: Optional[Node] = None):
        pass

I get an error: Node is not defined from Optional[Node].

Does this answer your question? [How to create object inside the same python class](https://stackoverflow.com/questions/31108208/how-to-create-object-inside-the-same-python-class) — Amir Shamsi, Apr 16 '22 at 14:31

score 1 · Accepted Answer · answered Apr 16 '22 at 14:32

Check out How do I type hint a method with the type of the enclosing class?. You can either:

Use from __future__ import annotations at the top of the file and then use Node as you did.
Use a string ('Node'), see @Diptangsu Goswami's comment.
From Python 3.11 and onwards (not out yet) you will be able to use from typing import Self and then Optional[Self].

Referencing this custom class with typing

1 Answers1