[issue45972] typing.NamedTuple with default arguments without type annotations is falsy

Erik Montnemery added the comment: Maybe something like this: diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst index 735d477db4..8de913d8db 100644 --- a/Doc/library/typing.rst +++ b/Doc/library/typing.rst @@ -1291,7 +1291,8 @@ These are not used in annotations. They are building

[issue45972] typing.NamedTuple with default arguments without type annotations is falsy

Alex Waygood added the comment: To me, the fact that NamedTuple uses class attributes to provide field defaults feels like an implementation detail that is only relevant to an unusual edge case. Where do you think the documentation should be improved, and what is your suggested wording? :)

[issue45972] typing.NamedTuple with default arguments without type annotations is falsy

Erik Montnemery added the comment: I think elaborating in the documentation that only annotated attributes make it to the underlying namedtuple() would be helpful, it's not obvious that they are instead just class attributes. -- ___ Python

[issue45972] typing.NamedTuple with default arguments without type annotations is falsy

Alex Waygood added the comment: I agree that prohibiting zero-length NamedTuples seems like a bad idea, and also agree that this probably doesn't need to be documented. The behaviour here definitely looks weird at first glance, but it's probably not a good idea to tamper with the __bool__()

[issue45972] typing.NamedTuple with default arguments without type annotations is falsy

Eric V. Smith added the comment: I don't think we'd want to prohibit zero-length namedtuples (or NamedTuples). I've used them, especially when I'm dynamically creating them. This is just a side effect of how Python works. I don't think there's anything to do here, except maybe mention it in

[issue45972] typing.NamedTuple with default arguments without type annotations is falsy

Spencer Brown added the comment: What's happening is that typing.NamedTuple ignores non-annotated attributes entirely when computing the names it passes along to namedtuple(), so here "a" is just a class attribute. You're accessing it from there, but the tuple itself is entirely empty.

[issue45972] typing.NamedTuple with default arguments without type annotations is falsy

Change by Alex Waygood : -- components: +Library (Lib) nosy: +AlexWaygood, gvanrossum, kj ___ Python tracker ___ ___

[issue45972] typing.NamedTuple with default arguments without type annotations is falsy

New submission from Erik Montnemery : typing.NamedTuple behaves in surprising ways when it has default arguments which lack type annotations: >>> from typing import NamedTuple >>> class MyTuple(NamedTuple): ... a = 1000 ... >>> tmp = MyTuple() >>> tmp.a 1000 >>> len(tmp) 0 >>> bool(tmp)