Add ast.Constant

Issue #26146: Add a new kind of AST node: ast.Constant. It can be used by
external AST optimizers, but the compiler does not emit directly such node.

An optimizer can replace the following AST nodes with ast.Constant:

* ast.NameConstant: None, False, True
* ast.Num: int, float, complex
* ast.Str: str
* ast.Bytes: bytes
* ast.Tuple if items are constants too: tuple
* frozenset

Update code to accept ast.Constant instead of ast.Num and/or ast.Str:

* compiler
* docstrings
* ast.literal_eval()
* Tools/parser/unparse.py

Lib/test/test_ast.py

@@ -1,7 +1,8 @@
+import ast
+import dis
 import os
 import sys
 import unittest
-import ast
 import weakref
 
 from test import support
@@ -933,6 +934,123 @@ class ASTValidatorTests(unittest.TestCase):
             compile(mod, fn, "exec")
 
 
+class ConstantTests(unittest.TestCase):
+    """Tests on the ast.Constant node type."""
+
+    def compile_constant(self, value):
+        tree = ast.parse("x = 123")
+
+        node = tree.body[0].value
+        new_node = ast.Constant(value=value)
+        ast.copy_location(new_node, node)
+        tree.body[0].value = new_node
+
+        code = compile(tree, "<string>", "exec")
+
+        ns = {}
+        exec(code, ns)
+        return ns['x']
+
+    def test_singletons(self):
+        for const in (None, False, True, Ellipsis, b'', frozenset()):
+            with self.subTest(const=const):
+                value = self.compile_constant(const)
+                self.assertIs(value, const)
+
+    def test_values(self):
+        nested_tuple = (1,)
+        nested_frozenset = frozenset({1})
+        for level in range(3):
+            nested_tuple = (nested_tuple, 2)
+            nested_frozenset = frozenset({nested_frozenset, 2})
+        values = (123, 123.0, 123j,
+                  "unicode", b'bytes',
+                  tuple("tuple"), frozenset("frozenset"),
+                  nested_tuple, nested_frozenset)
+        for value in values:
+            with self.subTest(value=value):
+                result = self.compile_constant(value)
+                self.assertEqual(result, value)
+
+    def test_assign_to_constant(self):
+        tree = ast.parse("x = 1")
+
+        target = tree.body[0].targets[0]
+        new_target = ast.Constant(value=1)
+        ast.copy_location(new_target, target)
+        tree.body[0].targets[0] = new_target
+
+        with self.assertRaises(ValueError) as cm:
+            compile(tree, "string", "exec")
+        self.assertEqual(str(cm.exception),
+                         "expression which can't be assigned "
+                         "to in Store context")
+
+    def test_get_docstring(self):
+        tree = ast.parse("'docstring'\nx = 1")
+        self.assertEqual(ast.get_docstring(tree), 'docstring')
+
+        tree.body[0].value = ast.Constant(value='constant docstring')
+        self.assertEqual(ast.get_docstring(tree), 'constant docstring')
+
+    def get_load_const(self, tree):
+        # Compile to bytecode, disassemble and get parameter of LOAD_CONST
+        # instructions
+        co = compile(tree, '<string>', 'exec')
+        consts = []
+        for instr in dis.get_instructions(co):
+            if instr.opname == 'LOAD_CONST':
+                consts.append(instr.argval)
+        return consts
+
+    @support.cpython_only
+    def test_load_const(self):
+        consts = [None,
+                  True, False,
+                  124,
+                  2.0,
+                  3j,
+                  "unicode",
+                  b'bytes',
+                  (1, 2, 3)]
+
+        code = '\n'.join(map(repr, consts))
+        code += '\n...'
+
+        code_consts = [const for const in consts
+                       if (not isinstance(const, (str, int, float, complex))
+                           or isinstance(const, bool))]
+        code_consts.append(Ellipsis)
+        # the compiler adds a final "LOAD_CONST None"
+        code_consts.append(None)
+
+        tree = ast.parse(code)
+        self.assertEqual(self.get_load_const(tree), code_consts)
+
+        # Replace expression nodes with constants
+        for expr_node, const in zip(tree.body, consts):
+            assert isinstance(expr_node, ast.Expr)
+            new_node = ast.Constant(value=const)
+            ast.copy_location(new_node, expr_node.value)
+            expr_node.value = new_node
+
+        self.assertEqual(self.get_load_const(tree), code_consts)
+
+    def test_literal_eval(self):
+        tree = ast.parse("1 + 2")
+        binop = tree.body[0].value
+
+        new_left = ast.Constant(value=10)
+        ast.copy_location(new_left, binop.left)
+        binop.left = new_left
+
+        new_right = ast.Constant(value=20)
+        ast.copy_location(new_right, binop.right)
+        binop.right = new_right
+
+        self.assertEqual(ast.literal_eval(binop), 30)
+
+
 def main():
     if __name__ != '__main__':
         return