gh-104799: Default missing lists in AST to the empty list (#104834)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Lib/test/test_ast.py

@@ -1591,6 +1591,8 @@ class ASTValidatorTests(unittest.TestCase):
         f = ast.FunctionDef("x", a, [ast.Pass()], [],
                             ast.Name("x", ast.Store()), None, [])
         self.stmt(f, "must have Load context")
+        f = ast.FunctionDef("x", ast.arguments(), [ast.Pass()])
+        self.stmt(f)
         def fac(args):
             return ast.FunctionDef("x", args, [ast.Pass()], [], None, None, [])
         self._check_arguments(fac, self.stmt)

Misc/NEWS.d/next/Core and Builtins/2023-05-31-08-10-59.gh-issue-104799.8kDWti.rst

@@ -0,0 +1,4 @@
+Attributes of :mod:`ast` nodes that are lists now default to the empty list
+if omitted. This means that some code that previously raised
+:exc:`TypeError` when the AST node was used will now proceed with the empty
+list instead. Patch by Jelle Zijlstra.

Parser/asdl_c.py

@@ -632,6 +632,15 @@ class Obj2ModVisitor(PickleVisitor):
         self.emit(line % field.name, depth)
         self.emit("return 1;", depth+1)
         self.emit("}", depth)
+        if field.seq:
+            self.emit("if (tmp == NULL) {", depth)
+            self.emit("tmp = PyList_New(0);", depth+1)
+            self.emit("if (tmp == NULL) {", depth+1)
+            self.emit("return 1;", depth+2)
+            self.emit("}", depth+1)
+            self.emit("}", depth)
+            self.emit("{", depth)
+        else:
             if not field.opt:
                 self.emit("if (tmp == NULL) {", depth)
                 message = "required field \\\"%s\\\" missing from %s" % (field.name, name)