Python 3.9 was released this week, and with it come a few incremental features. But my favorite feature isn’t just in Python 3.9—it’s usable in 3.7 and up.
Type hinting generics
If you’re using type annotations in Python, you’ve surely had to write from typing import List before. And you’ve probably made the mistake of writing def f(d: dict[str, int]) and getting a TypeError.
No more! Starting in Python 3.9, the following code will all just work, thanks to PEP 585:
def f(d: dict[str, int]):
...
x: list[int] = ...
y: tuple[int, int] = ...
No more annoying imports from typing! No more (totally understandable) questions from beginners about the difference between list and List.
To be clear, I understand why the typing module was designed this way. Adding List in typing made type hinting compatible with previous versions of Python. And more generally, the design of annotations and type hinting were constrained by not wanting to change the core language without knowing how the feature would be received.
With all this said … it was still annoying. 🙂
Enter: __future__
Despite this great addition, I’m not going to be using Python 3.9 yet. Maybe for a new project, but I don’t have any planned. And we certainly won’t be upgrading our codebase at work for a while.
My favorite part of Python 3.9, though, is that this is available from 3.7 onwards. This is all thanks to __future__.
Last week, this code failed on 3.7:
def f(x: list[int]):
return sum(x)
f([1, 2, 3])
But today, adding a from __future__ import annotations at the top, it works perfectly fine.
Goodbye, from typing import List!