Hey, thanks a lot for pointing this out! Yet another reason to use data classes vs. named tuples.
Looks like just plain class level attribute declaration also works:
class Node:
child: 'Node'
I wonder why `typing.NamedTuple` is unique as far as it not working. I know they don't use `eval` and templating to create it anymore. From looking at the code [0], they're using metaclass / __new__. But other than the fact that it uses metaclasses, I'm not sure why it'd have an error. Obviously there's cycle handling in mypy, otherwise none of the examples would work. If I use an example with a metaclass, that also typechecks. So, it's not metaclasses that trigger the error.
class NodeBase(type):
def __new__(cls, name, bases, attrs):
return super().__new__(cls, name, bases, attrs)
class Node(metaclass=NodeBase):
child: 'Node' # works
Edit: it looks like there is now work going into this and other cases where recursive types don't work [1][2]. For example: `Callback = typing.Callable[[str], Callback]` and `Foo = Union[str, List['Foo']]`.
Looks like just plain class level attribute declaration also works:
I wonder why `typing.NamedTuple` is unique as far as it not working. I know they don't use `eval` and templating to create it anymore. From looking at the code [0], they're using metaclass / __new__. But other than the fact that it uses metaclasses, I'm not sure why it'd have an error. Obviously there's cycle handling in mypy, otherwise none of the examples would work. If I use an example with a metaclass, that also typechecks. So, it's not metaclasses that trigger the error. Edit: it looks like there is now work going into this and other cases where recursive types don't work [1][2]. For example: `Callback = typing.Callable[[str], Callback]` and `Foo = Union[str, List['Foo']]`.[0] https://github.com/python/cpython/blob/3.8/Lib/typing.py#L15...
[1] https://github.com/python/mypy/issues/731
[2] https://github.com/python/mypy/issues/6204