Wrong `Python.asdl` type for `keys` in `Dict` literals

[Demonstrandum]

In the Python.asdl syntax description, the file asserts that Dict literal keys have type expr*. This is false. keys can be optional.

In the ast documentation for Python:

• https://docs.python.org/3/library/ast.html#ast.Dict it is made clear that:

When doing dictionary unpacking using dictionary literals the expression to be expanded goes in the values list, with a None at the corresponding position in keys.

Hence, keys is really a expr?* and not a expr*.

Linked PRs

• gh-131421: fix ASDL grammar for Dict to have an expr?* keys field #131419

[picnixz]

In the ADSL grammar, I don't think ?* is allowed. The ? stands for "optional", namely something that can be omitted, while * stands for "0 or more" arguments, given as a list, so IIRC, it represents any kind of lists. Here, the ASDL entry for Dict only means that keys is a list, with zero or more expressions.

cc @JelleZijlstra

[Demonstrandum]

In the ADSL grammar, I don't think ?* is allowed. The ? stands for "optional", namely something that can be omitted, while * stands for "0 or more" arguments, given as a list, so IIRC, it represents any kind of lists. Here, the ASDL entry for Dict only means that keys is a list, with zero or more expressions.

cc @JelleZijlstra

Right, but there aren't 0 or more expressions, but rather 0 or more optional expressions, ie an expression can be None instead of an actual expr

[picnixz]

None is a valid expression (it's a constant):

>>> ast.parse('None')
Module(body=[Expr(value=Constant(value=None, kind=None))], type_ignores=[])

[picnixz]

Ah I understand your point now. You mean that the None is not a true None but a sentinel None.