pythongh-105858: Improve AST node constructors

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>

Known problems:
- Subclasses of AST nodes don't work properly, because we don't look up __annotations__ on the
  right class.
- Unpickling throws DeprecationWarnings, probably because of how we construct the unpickled
  object.

Need to think more about how to handle those cases.

Lib/test/test_ast.py

@@ -495,9 +495,17 @@ def test_arguments(self):
         x = ast.arguments()
         self.assertEqual(x._fields, ('posonlyargs', 'args', 'vararg', 'kwonlyargs',
                                      'kw_defaults', 'kwarg', 'defaults'))
-
-        with self.assertRaises(AttributeError):
-            x.args
+        self.assertEqual(x.__annotations__, {
+            'posonlyargs': list[ast.arg],
+            'args': list[ast.arg],
+            'vararg': ast.arg | None,
+            'kwonlyargs': list[ast.arg],
+            'kw_defaults': list[ast.expr],
+            'kwarg': ast.arg | None,
+            'defaults': list[ast.expr],
+        })
+
+        self.assertEqual(x.args, [])
         self.assertIsNone(x.vararg)
 
         x = ast.arguments(*range(1, 8))
@@ -573,15 +581,22 @@ def test_classattrs_deprecated(self):
 
         self.assertEqual([str(w.message) for w in wlog], [
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
+            "Constant.__init__ missing 1 required positional argument: 'value'. This will become "
+            'an error in Python 3.15.',
             'Attribute n is deprecated and will be removed in Python 3.14; use value instead',
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
             'Attribute n is deprecated and will be removed in Python 3.14; use value instead',
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
+            "Constant.__init__ missing 1 required positional argument: 'value'. This will become "
+            'an error in Python 3.15.',
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
             'Attribute n is deprecated and will be removed in Python 3.14; use value instead',
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
+            "Constant.__init__ got an unexpected keyword argument 'foo'. Support for "
+            'arbitrary keyword arguments is deprecated and will be removed in Python '
+            '3.15.',
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
             'Attribute n is deprecated and will be removed in Python 3.14; use value instead',
             'ast.Num is deprecated and will be removed in Python 3.14; use ast.Constant instead',
@@ -2828,6 +2843,23 @@ def visit_Call(self, node: ast.Call):
         self.assertASTTransformation(PrintToLog, code, expected)
 
 
+class ASTConstructorTests(unittest.TestCase):
+    """Test the autogenerated constructors for AST nodes."""
+
+    def test_FunctionDef(self):
+        args = ast.arguments()
+        self.assertEqual(args.args, [])
+        self.assertEqual(args.posonlyargs, [])
+        with self.assertWarnsRegex(DeprecationWarning,
+                                   r"FunctionDef\.__init__ missing 1 required positional argument: 'name'"):
+            node = ast.FunctionDef(args=args)
+        self.assertFalse(hasattr(node, "name"))
+        self.assertEqual(node.decorator_list, [])
+        node = ast.FunctionDef(name='foo', args=args)
+        self.assertEqual(node.name, 'foo')
+        self.assertEqual(node.decorator_list, [])
+
+
 @support.cpython_only
 class ModuleStateTests(unittest.TestCase):
     # bpo-41194, bpo-41261, bpo-41631: The _ast module uses a global state.

Misc/NEWS.d/next/Core and Builtins/2023-06-16-21-29-06.gh-issue-105858.Q7h0EV.rst

@@ -0,0 +1,7 @@
+Improve the constructors for :mod:`ast` nodes. Arguments of list types now
+default to an empty list if omitted. AST nodes now have an
+``__annotations__`` attribute with the expected types of their attributes.
+Passing unrecognized extra arguments to AST nodes is deprecated and will
+become an error in Python 3.15. Omitting a required argument to an AST node
+is deprecated and will become an error in Python 3.15. Patch by Jelle
+Zijlstra.

Parser/asdl_c.py

@@ -15,6 +15,13 @@
 MAX_COL = 80
 AUTOGEN_MESSAGE = "// File automatically generated by {}.\n\n"
 
+builtin_type_to_c_type = {
+    "identifier": "PyUnicode_Type",
+    "string": "PyUnicode_Type",
+    "int": "PyLong_Type",
+    "constant": "PyBaseObject_Type",
+}
+
 def get_c_type(name):
     """Return a string for the C name of the type.
 
@@ -764,6 +771,65 @@ def visitConstructor(self, cons, name):
             self.emit("};",0)
 
 
+class AnnotationsVisitor(PickleVisitor):
+    def visitModule(self, mod):
+        self.file.write(textwrap.dedent('''
+            static int
+            add_ast_annotations(struct ast_state *state)
+            {
+                bool cond;
+        '''))
+        for dfn in mod.dfns:
+            self.visit(dfn)
+        self.file.write(textwrap.dedent('''
+                return 1;
+            }
+        '''))
+
+    def visitProduct(self, prod, name):
+        self.emit_annotations(name, prod.fields)
+
+    def visitSum(self, sum, name):
+        for t in sum.types:
+            self.visitConstructor(t, name)
+
+    def visitConstructor(self, cons, name):
+        self.emit_annotations(cons.name, cons.fields)
+
+    def emit_annotations(self, name, fields):
+        self.emit(f"PyObject *{name}_annotations = PyDict_New();", 1)
+        self.emit(f"if (!{name}_annotations) return 0;", 1)
+        for field in fields:
+            self.emit("{", 1)
+            if field.type in builtin_type_to_c_type:
+                self.emit(f"PyObject *type = (PyObject *)&{builtin_type_to_c_type[field.type]};", 2)
+            else:
+                self.emit(f"PyObject *type = state->{field.type}_type;", 2)
+            if field.opt:
+                self.emit("type = _Py_union_type_or(type, Py_None);", 2)
+                self.emit("cond = type != NULL;", 2)
+                self.emit_annotations_error(name, 2)
+            elif field.seq:
+                self.emit("type = Py_GenericAlias((PyObject *)&PyList_Type, type);", 2)
+                self.emit("cond = type != NULL;", 2)
+                self.emit_annotations_error(name, 2)
+            else:
+                self.emit("Py_INCREF(type);", 2)
+            self.emit(f"cond = PyDict_SetItemString({name}_annotations, \"{field.name}\", type) == 0;", 2)
+            self.emit("Py_DECREF(type);", 2)
+            self.emit_annotations_error(name, 2)
+            self.emit("}", 1)
+        self.emit(f'cond = PyObject_SetAttrString(state->{name}_type, "__annotations__", {name}_annotations) == 0;', 1)
+        self.emit(f"Py_DECREF({name}_annotations);", 1)
+        self.emit("if (!cond) return 0;", 1)
+
+    def emit_annotations_error(self, name, depth):
+        self.emit("if (!cond) {", depth)
+        self.emit(f"Py_DECREF({name}_annotations);", depth + 1)
+        self.emit("return 0;", depth + 1)
+        self.emit("}", depth)
+
+
 class PyTypesVisitor(PickleVisitor):
 
     def visitModule(self, mod):
@@ -812,7 +878,7 @@ def visitModule(self, mod):
 
     Py_ssize_t i, numfields = 0;
     int res = -1;
-    PyObject *key, *value, *fields;
+    PyObject *key, *value, *fields, *remaining_fields = NULL;
     if (_PyObject_LookupAttr((PyObject*)Py_TYPE(self), state->_fields, &fields) < 0) {
         goto cleanup;
     }
@@ -821,6 +887,13 @@ def visitModule(self, mod):
         if (numfields == -1) {
             goto cleanup;
         }
+        remaining_fields = PySet_New(fields);
+    }
+    else {
+        remaining_fields = PySet_New(NULL);
+    }
+    if (remaining_fields == NULL) {
+        goto cleanup;
     }
 
     res = 0; /* if no error occurs, this stays 0 to the end */
@@ -840,6 +913,11 @@ def visitModule(self, mod):
             goto cleanup;
         }
         res = PyObject_SetAttr(self, name, PyTuple_GET_ITEM(args, i));
+        if (PySet_Discard(remaining_fields, name) < 0) {
+            res = -1;
+            Py_DECREF(name);
+            goto cleanup;
+        }
         Py_DECREF(name);
         if (res < 0) {
             goto cleanup;
@@ -852,29 +930,101 @@ def visitModule(self, mod):
             if (contains == -1) {
                 res = -1;
                 goto cleanup;
-            } else if (contains == 1) {
-                Py_ssize_t p = PySequence_Index(fields, key);
+            }
+            else if (contains == 1) {
+                int p = PySet_Discard(remaining_fields, key);
                 if (p == -1) {
                     res = -1;
                     goto cleanup;
                 }
-                if (p < PyTuple_GET_SIZE(args)) {
+                if (p == 0) {
                     PyErr_Format(PyExc_TypeError,
                         "%.400s got multiple values for argument '%U'",
                         Py_TYPE(self)->tp_name, key);
                     res = -1;
                     goto cleanup;
                 }
             }
+            else if (
+                PyUnicode_CompareWithASCIIString(key, "lineno") != 0 &&
+                PyUnicode_CompareWithASCIIString(key, "col_offset") != 0 &&
+                PyUnicode_CompareWithASCIIString(key, "end_lineno") != 0 &&
+                PyUnicode_CompareWithASCIIString(key, "end_col_offset") != 0
+            ) {
+                if (PyErr_WarnFormat(
+                    PyExc_DeprecationWarning, 1,
+                    "%.400s.__init__ got an unexpected keyword argument '%U'. "
+                    "Support for arbitrary keyword arguments is deprecated "
+                    "and will be removed in Python 3.15.",
+                    Py_TYPE(self)->tp_name, key
+                ) < 0) {
+                    goto cleanup;
+                }
+            }
             res = PyObject_SetAttr(self, key, value);
             if (res < 0) {
                 goto cleanup;
             }
         }
     }
+    Py_ssize_t size = PySet_Size(remaining_fields);
+    PyObject *annotations = NULL, *remaining_list = NULL;
+    if (size > 0) {
+        if (!_PyObject_LookupAttr((PyObject*)Py_TYPE(self), &_Py_ID(__annotations__), &annotations)) {
+            res = -1;
+            goto cleanup;
+        }
+        remaining_list = PySequence_List(remaining_fields);
+        if (!remaining_list) {
+            goto set_remaining_cleanup;
+        }
+        for (Py_ssize_t i = 0; i < size; i++) {
+            PyObject *name = PyList_GET_ITEM(remaining_list, i);
+            PyObject *type = PyDict_GetItemWithError(annotations, name);
+            if (!type) {
+                if (!PyErr_Occurred()) {
+                    PyErr_SetObject(PyExc_KeyError, name);
+                }
+                goto set_remaining_cleanup;
+            }
+            if (_PyUnion_Check(type)) {
+                // optional field
+                // do nothing, we'll have set a None default on the class
+            }
+            else if (Py_IS_TYPE(type, &Py_GenericAliasType)) {
+                // list field
+                PyObject *empty = PyList_New(0);
+                if (!empty) {
+                    goto set_remaining_cleanup;
+                }
+                res = PyObject_SetAttr(self, name, empty);
+                Py_DECREF(empty);
+                if (res < 0) {
+                    goto set_remaining_cleanup;
+                }
+            }
+            else {
+                // simple field (e.g., identifier)
+                if (PyErr_WarnFormat(
+                    PyExc_DeprecationWarning, 1,
+                    "%.400s.__init__ missing 1 required positional argument: '%U'. "
+                    "This will become an error in Python 3.15.",
+                    Py_TYPE(self)->tp_name, name
+                ) < 0) {
+                    goto cleanup;
+                }
+            }
+        }
+    }
   cleanup:
     Py_XDECREF(fields);
+    Py_XDECREF(remaining_fields);
     return res;
+  set_remaining_cleanup:
+    Py_XDECREF(remaining_list);
+    Py_XDECREF(annotations);
+    res = -1;
+    goto cleanup;
 }
 
 /* Pickling support */
@@ -1123,6 +1273,9 @@ def visitModule(self, mod):
         for dfn in mod.dfns:
             self.visit(dfn)
         self.file.write(textwrap.dedent('''
+                if (!add_ast_annotations(state)) {
+                    return 0;
+                }
                 state->recursion_depth = 0;
                 state->recursion_limit = 0;
                 state->initialized = 1;
@@ -1542,6 +1695,7 @@ def generate_module_def(mod, metadata, f, internal_h):
         #include "pycore_ceval.h"         // _Py_EnterRecursiveCall
         #include "pycore_interp.h"        // _PyInterpreterState.ast
         #include "pycore_pystate.h"       // _PyInterpreterState_GET()
+        #include "pycore_unionobject.h"   // _Py_union_type_or
         #include "structmember.h"
         #include <stddef.h>
 
@@ -1651,6 +1805,7 @@ def write_source(mod, metadata, f, internal_h_file):
     v = ChainOfVisitors(
         SequenceConstructorVisitor(f),
         PyTypesDeclareVisitor(f),
+        AnnotationsVisitor(f),
         PyTypesVisitor(f),
         Obj2ModPrototypeVisitor(f),
         FunctionVisitor(f),