How to type annotate a static field of a class that references the same class?

Question

I am maintaining a collection of created objects within class:

class Foo:
    all_of_us: = []

    def __init__(self):
        Foo.all_of_us.append(self)

Which fails if I try to type annotate:

import typing

class Foo:
    all_of_us: typing.List[Foo] = []

With NameError: name 'Foo' is not defined.

What is the right way to go about it?

The upcoming Python 3.11 will have a `Self` type for this use https://peps.python.org/pep-0673/ — Håken Lid, Jun 02 '22 at 22:51

score 3 · Accepted Answer · answered Jun 02 '22 at 22:46

you have two choices :

if you are using a recent version of python, you can use the annotations module from __future__ (that will store your annotations directly as strings without having to do it manually)

from __future__ import annotations
from typing import List

class Foo:
    all_of_us: List[Foo] = []

if you have an older version, simply put our class type inside a string manually:

from typing import List

class Foo:
    all_of_us: List['Foo'] = []

How to type annotate a static field of a class that references the same class?

1 Answers1