Recursive class typing in python

Question

I'm looking for typing a class with recursive field

I try the following code:

class Heh:
  child_hehs: list[Heh]

but i got following error

NameError: name 'Heh' is not defined

so I try another code:

class MetaHeh(Heh):
  pass

class Heh:
  child_hehs: list[MetaHeh]

and i got following error again:

NameError: name 'Heh' is not defined

How can i implement this code with typing?

Answer 1

Use the name of the recursive type in its declaration. For example:

class Heh:
  child_hehs: list['Heh']

---

If you can use python 3.7.0b1 and above you can use Postponed Evaluation of Annotations.

from __future__ import annotations 
class Heh:     
    child_hehs: list[Heh]

This feature is expected to become part of the language in python 3.10 (in other words, without __future__ import).