Improve the constructors of AST nodes

[JelleZijlstra]

Currently, the constructors for AST nodes accept arbitrary keyword arguments and don't enforce any value:

>>> node=ast.FunctionDef(what="is this")
>>> node.what
'is this'
>>> node.name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'FunctionDef' object has no attribute 'name'

Problems with the current situation:

• To make a correct AST node, you have to pass every attribute, including ones that could sensibly default to an empty list
• You cannot rely on attributes like name being present
• It's possible to create useless, broken AST nodes like the above
• Introspection tools can't easily tell what the signature of the constructor is supposed to be
• Adding new fields to an AST node causes various compatibility issues (PEP-695: Potentially breaking changes made to __match_args__ attributes of AST nodes #104799)

Proposed solution for 3.13:

• Default all fields to some sensible value if they are not provided to the constructor: an empty list for list fields, and None for other fields.
• Emit a DeprecationWarning if the constructor is passed arguments that it doesn't recognize (e.g., what above). In 3.15, this will raise an error.
• Add a __text_signature__ to the AST classes indicating the expected signature.

Linked PRs

• gh-105858: Improve AST node constructors #105880
• gh-105858: Expose some union-related objects as internal APIs #116025

[furkanonder]

CC: @isidentical

[brandtbucher]

Yes please! This has been such a pain point for me.

One note: we should probably only default optional fields (?) to None, and require everything else.

[JelleZijlstra]

One note: we should probably only default optional fields (?) to None, and require everything else.

Agree, I ended up doing that in my draft PR. Omitting a required field (e.g. FunctionDef.name) will raise a DeprecationWarning, as will passing a bogus field.

[JelleZijlstra]

Merged!