Question about ListView implementation

[liancheng]

I'm trying to build a radio input widget using the ListView widget as an example and got puzzled by the following part of the ListView.Selected implementation:

class ListView(Vertical, can_focus=True, can_focus_children=False):
    ...

    class Selected(Message, bubble=True):
        ...
        def __init__(self, sender: ListView, item: ListItem) -> None:   # <-- NOTE
            super().__init__(sender)
            self.item: ListItem = item

Please note that Selected.__init__ referenced ListView as part of the argument list. While adding a custom event for my radio input widget in a similar way, I got a "name 'RadioInput' not defined" error. The following simplified snippet reproduces this issue:

>>> class A:
...   class B:
...     def __init__(self, a: A):
...       pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in A
  File "<stdin>", line 3, in B
NameError: name 'A' is not defined

I'm not a Python expert but I guess this makes some sense since the definition of the outer class A is incomplete when A is referenced in the argument list of B.__init__? However, why did it work in the ListView case?

(BTW, I mitigated this issue by changing the type of sender to MessageTarget instead of the concrete widget class.)

[willmcgugan]

Try this at the top of your file:

from future import __annotations__

[liancheng]

Thank you, it worked! Although I guess you meant

from __future__ import annotations

Wondered why for a while until I remembered that Python type hints are annotations as well. For future reference, the "Note" in this section explains why https://docs.python.org/3.10/library/typing.html#typing.TYPE_CHECKING