namedtuple defaults in stubs

[hmc-cs-mdrissi]

class Foo(NamedTuple):
  x: bool = False

produces an error message similar to,

  File "/Users/pa-loaner/Snapchat/Dev/.venvs/typeshed/lib/python3.9/site-packages/pytype/ast/visitor.py", line 55, in _call_visitor
    return visitor(node)
  File "/Users/pa-loaner/Snapchat/Dev/.venvs/typeshed/lib/python3.9/site-packages/pytype/pyi/parser.py", line 477, in visit_AnnAssign
    raise ParseError(msg)
pytype.pyi.types.ParseError:   File: "stubs/tensorflow/tensorflow/io/__init__.pyi", line 56
    default_value: _TensorCompatible | None = None
   ^
ParseError: Default value for default_value: typing.Union can only be '...', got None

I think it then leads to various noise errors elsewhere due to parse error. Originally hit here by running pytype_test.py stubs/tensorflow/io but you can run pytype_test.py stubs/tensorflow/toy and add a file in toy with just example at top to trigger this error. The error looks to be specific to stub files and normal .py files are fine.

edit: Looking a bit more at code/stacktrace I'm unsure this is unique to namedtuples although probably most common to see default with them in class body.

[hmc-cs-mdrissi]

I'm able to take a stab at this, but unsure which values you want to allow. typeshed exact lint rule is a bit complex and mostly motivated by what can stubtest/static ast checking with flake8 handle and isn't too "complex". Some of which is very heuristicy (long tuples/strings being blocked). I'm unsure what mypy/pyright's valid stub default values are.

The easiest rule is just allow everything as valid stub defaults. Alternatively allowlist only some types/constants?

At the least default value should not be raising a ParseError that can't be type ignored/affects other files.

edit: Or maybe same rule/behavior function defaults have should be used?

edit 2: Based on this pr my guess is to just not process defaults at all for .pyi files and delete some of this logic . At same I see the logic for x: Final default values only allows Literals, but typeshed default value allows non literals too if "simple" enough like constant tuple/dict.

[rchen152]

Looks like "literals and constants" are supposed to be allowed as default values: https://typing.readthedocs.io/en/latest/source/stubs.html#functions-and-methods. I could've sworn that doc did not used to say that, but I can implement it.

edit: I checked the version history, and that was indeed there all along ^^; Surprised that it's taken this long for this issue to crop up.