How to declare python dataclass member field same as the dataclass type

Question

How can I have a member field of a dataclass same as the class name in python3.7+ ? I am trying to define a class like so (which can be done in Java or C++) -- which might be used as a class for LinkedList node

@dataclass
class Node:
  val:str
  next:Node
  prev:Node

However, all I get is NameError: name 'Node' is not defined. What should be the correct way to have self referential member variables in dataclasses

Does [this](https://stackoverflow.com/questions/9305751/how-to-force-ensure-class-attributes-are-a-specific-type) answer your question? — bhristov, Jun 22 '20 at 19:15

Iain Shelvington · Accepted Answer · 2020-06-22T20:30:34.453

10

You need to add the following import to your file

from __future__ import annotations

This enables deferred annotations, which are proposed in PEP 563. Deferred annotations allow you to reference a class that is not yet defined, in your example the Node class is not defined when you are still in it's body

edited Jun 22 '20 at 20:30

answered Jun 22 '20 at 19:20

Iain Shelvington

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score 7 · Answer 2 · answered Nov 09 '20 at 17:19

Another way is to use a TypeVar:

from dataclasses import dataclass
from typing import TypeVar

Node = TypeVar("Node")

@dataclass
class Node:
  val:  str
  next: Node
  prev: Node

This is also useful when dealing with annotations for methods of a class.

How to declare python dataclass member field same as the dataclass type

2 Answers2