Different behaviour of dataclass default_factory to generate list

Question

I'm quite new to Python so please have me excused if this question contain some newbie misunderstandings, but I've failed to google the answer for this:

On my personal laptop running Python 3.9.7 on Windows 11 this code is working without errors.

from dataclasses import dataclass, field


@dataclass
class SomeDataClass:
    somelist: list[str] = field(default_factory=lambda:['foo', 'bar'])

if __name__ == '__main__':
    instance = SomeDataClass()
    print(instance)

But when at work running Python 3.8.5 on Windows 10 I get the following error:

File "c:\...\test_dataclass.py", line 13, in SomeDataClass somelist: list[str] = field(default_factory=lambda:['foo', 'bar'])

TypeError: 'type' object is not subscriptable

I'd like to understand why this behaves differently and what I could do to make it work.

I would expect dataclasses to behave similarly on both computers.

Answer 1

You have already intuited the reason: this is a new feature in version 3.9. You can see it in the What's New article for 3.9 here.

This feature is available in version 3.8 as well, but it is not enabled by default. You can enable it in your code by including this import:

from __future__ import annotations