gh-105858: Improve AST node constructors (#105880)

Demonstration:

>>> ast.FunctionDef.__annotations__
{'name': <class 'str'>, 'args': <class 'ast.arguments'>, 'body': list[ast.stmt], 'decorator_list': list[ast.expr], 'returns': ast.expr | None, 'type_comment': str | None, 'type_params': list[ast.type_param]}
>>> ast.FunctionDef()
<stdin>:1: DeprecationWarning: FunctionDef.__init__ missing 1 required positional argument: 'name'. This will become an error in Python 3.15.
<stdin>:1: DeprecationWarning: FunctionDef.__init__ missing 1 required positional argument: 'args'. This will become an error in Python 3.15.
<ast.FunctionDef object at 0x101959460>
>>> node = ast.FunctionDef(name="foo", args=ast.arguments())
>>> node.decorator_list
[]
>>> ast.FunctionDef(whatever="you want", name="x", args=ast.arguments())
<stdin>:1: DeprecationWarning: FunctionDef.__init__ got an unexpected keyword argument 'whatever'. Support for arbitrary keyword arguments is deprecated and will be removed in Python 3.15.
<ast.FunctionDef object at 0x1019581f0>

1 files changed, 15 insertions, 0 deletions

diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
index ca5a0e7..3a277d7 100644
--- a/Doc/whatsnew/3.13.rst
+++ b/Doc/whatsnew/3.13.rst
@@ -206,6 +206,21 @@ array
 ast
 ---
 
+* The constructors of node types in the :mod:`ast` module are now stricter
+  in the arguments they accept, and have more intuitive behaviour when
+  arguments are omitted.
+
+  If an optional field on an AST node is not included as an argument when
+  constructing an instance, the field will now be set to ``None``. Similarly,
+  if a list field is omitted, that field will now be set to an empty list.
+  (Previously, in both cases, the attribute would be missing on the newly
+  constructed AST node instance.)
+
+  If other arguments are omitted, a :exc:`DeprecationWarning` is emitted.
+  This will cause an exception in Python 3.15. Similarly, passing a keyword
+  argument that does not map to a field on the AST node is now deprecated,
+  and will raise an exception in Python 3.15.
+
 * :func:`ast.parse` now accepts an optional argument ``optimize``
   which is passed on to the :func:`compile` built-in. This makes it
   possible to obtain an optimized ``AST``.