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

14 files changed, 401 insertions, 44 deletions

diff --git a/Include/Python-ast.h b/Include/Python-ast.h
index 175e380..1ab376a 100644
--- a/Include/Python-ast.h
+++ b/Include/Python-ast.h
@@ -202,9 +202,9 @@ enum _expr_kind {BoolOp_kind=1, BinOp_kind=2, UnaryOp_kind=3, Lambda_kind=4,
                   Await_kind=12, Yield_kind=13, YieldFrom_kind=14,
                   Compare_kind=15, Call_kind=16, Num_kind=17, Str_kind=18,
                   FormattedValue_kind=19, JoinedStr_kind=20, Bytes_kind=21,
-                  NameConstant_kind=22, Ellipsis_kind=23, Attribute_kind=24,
-                  Subscript_kind=25, Starred_kind=26, Name_kind=27,
-                  List_kind=28, Tuple_kind=29};
+                  NameConstant_kind=22, Ellipsis_kind=23, Constant_kind=24,
+                  Attribute_kind=25, Subscript_kind=26, Starred_kind=27,
+                  Name_kind=28, List_kind=29, Tuple_kind=30};
 struct _expr {
     enum _expr_kind kind;
     union {
@@ -316,6 +316,10 @@ struct _expr {
         } NameConstant;
         
         struct {
+            constant value;
+        } Constant;
+        
+        struct {
             expr_ty value;
             identifier attr;
             expr_context_ty ctx;
@@ -567,6 +571,9 @@ expr_ty _Py_NameConstant(singleton value, int lineno, int col_offset, PyArena
                          *arena);
 #define Ellipsis(a0, a1, a2) _Py_Ellipsis(a0, a1, a2)
 expr_ty _Py_Ellipsis(int lineno, int col_offset, PyArena *arena);
+#define Constant(a0, a1, a2, a3) _Py_Constant(a0, a1, a2, a3)
+expr_ty _Py_Constant(constant value, int lineno, int col_offset, PyArena
+                     *arena);
 #define Attribute(a0, a1, a2, a3, a4, a5) _Py_Attribute(a0, a1, a2, a3, a4, a5)
 expr_ty _Py_Attribute(expr_ty value, identifier attr, expr_context_ty ctx, int
                       lineno, int col_offset, PyArena *arena);
diff --git a/Include/asdl.h b/Include/asdl.h
index 495153c..35e9fa1 100644
--- a/Include/asdl.h
+++ b/Include/asdl.h
@@ -6,6 +6,7 @@ typedef PyObject * string;
 typedef PyObject * bytes;
 typedef PyObject * object;
 typedef PyObject * singleton;
+typedef PyObject * constant;
 
 /* It would be nice if the code generated by asdl_c.py was completely
    independent of Python, but it is a goal the requires too much work
diff --git a/Lib/ast.py b/Lib/ast.py
index 0170472..156a1f2 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -35,6 +35,8 @@ def parse(source, filename='<unknown>', mode='exec'):
     return compile(source, filename, mode, PyCF_ONLY_AST)
 
 
+_NUM_TYPES = (int, float, complex)
+
 def literal_eval(node_or_string):
     """
     Safely evaluate an expression node or a string containing a Python
@@ -47,7 +49,9 @@ def literal_eval(node_or_string):
     if isinstance(node_or_string, Expression):
         node_or_string = node_or_string.body
     def _convert(node):
-        if isinstance(node, (Str, Bytes)):
+        if isinstance(node, Constant):
+            return node.value
+        elif isinstance(node, (Str, Bytes)):
             return node.s
         elif isinstance(node, Num):
             return node.n
@@ -62,24 +66,21 @@ def literal_eval(node_or_string):
                         in zip(node.keys, node.values))
         elif isinstance(node, NameConstant):
             return node.value
-        elif isinstance(node, UnaryOp) and \
-             isinstance(node.op, (UAdd, USub)) and \
-             isinstance(node.operand, (Num, UnaryOp, BinOp)):
+        elif isinstance(node, UnaryOp) and isinstance(node.op, (UAdd, USub)):
             operand = _convert(node.operand)
-            if isinstance(node.op, UAdd):
-                return + operand
-            else:
-                return - operand
-        elif isinstance(node, BinOp) and \
-             isinstance(node.op, (Add, Sub)) and \
-             isinstance(node.right, (Num, UnaryOp, BinOp)) and \
-             isinstance(node.left, (Num, UnaryOp, BinOp)):
+            if isinstance(operand, _NUM_TYPES):
+                if isinstance(node.op, UAdd):
+                    return + operand
+                else:
+                    return - operand
+        elif isinstance(node, BinOp) and isinstance(node.op, (Add, Sub)):
             left = _convert(node.left)
             right = _convert(node.right)
-            if isinstance(node.op, Add):
-                return left + right
-            else:
-                return left - right
+            if isinstance(left, _NUM_TYPES) and isinstance(right, _NUM_TYPES):
+                if isinstance(node.op, Add):
+                    return left + right
+                else:
+                    return left - right
         raise ValueError('malformed node or string: ' + repr(node))
     return _convert(node_or_string)
 
@@ -196,12 +197,19 @@ def get_docstring(node, clean=True):
     """
     if not isinstance(node, (AsyncFunctionDef, FunctionDef, ClassDef, Module)):
         raise TypeError("%r can't have docstrings" % node.__class__.__name__)
-    if node.body and isinstance(node.body[0], Expr) and \
-       isinstance(node.body[0].value, Str):
-        if clean:
-            import inspect
-            return inspect.cleandoc(node.body[0].value.s)
-        return node.body[0].value.s
+    if not(node.body and isinstance(node.body[0], Expr)):
+        return
+    node = node.body[0].value
+    if isinstance(node, Str):
+        text = node.s
+    elif isinstance(node, Constant) and isinstance(node.value, str):
+        text = node.value
+    else:
+        return
+    if clean:
+        import inspect
+        text = inspect.cleandoc(text)
+    return text
 
 
 def walk(node):
diff --git a/Lib/test/test_ast.py b/Lib/test/test_ast.py
index d3e6d35..6d6c9bd 100644
--- a/Lib/test/test_ast.py
+++ b/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
diff --git a/Misc/NEWS b/Misc/NEWS
index 676f2ca..6e1a5a0 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,10 @@ Release date: tba
 Core and Builtins
 -----------------
 
+- 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.
+
 - Issue #18018: Import raises ImportError instead of SystemError if a relative
   import is attempted without a known parent package.
 
diff --git a/Parser/Python.asdl b/Parser/Python.asdl
index 22775c6..aaddf61 100644
--- a/Parser/Python.asdl
+++ b/Parser/Python.asdl
@@ -1,4 +1,8 @@
--- ASDL's six builtin types are identifier, int, string, bytes, object, singleton
+-- ASDL's 7 builtin types are:
+-- identifier, int, string, bytes, object, singleton, constant
+--
+-- singleton: None, True or False
+-- constant can be None, whereas None means "no value" for object.
 
 module Python
 {
@@ -76,6 +80,7 @@ module Python
          | Bytes(bytes s)
          | NameConstant(singleton value)
          | Ellipsis
+         | Constant(constant value)
 
          -- the following expression can appear in assignment context
          | Attribute(expr value, identifier attr, expr_context ctx)
diff --git a/Parser/asdl.py b/Parser/asdl.py
index 121cdab..62f5c19 100644
--- a/Parser/asdl.py
+++ b/Parser/asdl.py
@@ -33,7 +33,8 @@ __all__ = [
 # See the EBNF at the top of the file to understand the logical connection
 # between the various node types.
 
-builtin_types = {'identifier', 'string', 'bytes', 'int', 'object', 'singleton'}
+builtin_types = {'identifier', 'string', 'bytes', 'int', 'object', 'singleton',
+                 'constant'}
 
 class AST:
     def __repr__(self):
diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index eedd89b..a81644d 100644
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -834,6 +834,7 @@ static PyObject* ast2obj_object(void *o)
     return (PyObject*)o;
 }
 #define ast2obj_singleton ast2obj_object
+#define ast2obj_constant ast2obj_object
 #define ast2obj_identifier ast2obj_object
 #define ast2obj_string ast2obj_object
 #define ast2obj_bytes ast2obj_object
@@ -871,6 +872,26 @@ static int obj2ast_object(PyObject* obj, PyObject** out, PyArena* arena)
     return 0;
 }
 
+static int obj2ast_constant(PyObject* obj, PyObject** out, PyArena* arena)
+{
+    if (obj == Py_None || obj == Py_True || obj == Py_False) {
+        /* don't increment the reference counter, Constant uses a borrowed
+         * reference, not a strong reference */
+        *out = obj;
+        return 0;
+    }
+
+    if (obj) {
+        if (PyArena_AddPyObject(arena, obj) < 0) {
+            *out = NULL;
+            return -1;
+        }
+        Py_INCREF(obj);
+    }
+    *out = obj;
+    return 0;
+}
+
 static int obj2ast_identifier(PyObject* obj, PyObject** out, PyArena* arena)
 {
     if (!PyUnicode_CheckExact(obj) && obj != Py_None) {
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index 07d9b3e..4dde11f 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -306,6 +306,10 @@ static char *NameConstant_fields[]={
     "value",
 };
 static PyTypeObject *Ellipsis_type;
+static PyTypeObject *Constant_type;
+static char *Constant_fields[]={
+    "value",
+};
 static PyTypeObject *Attribute_type;
 _Py_IDENTIFIER(attr);
 _Py_IDENTIFIER(ctx);
@@ -709,6 +713,7 @@ static PyObject* ast2obj_object(void *o)
     return (PyObject*)o;
 }
 #define ast2obj_singleton ast2obj_object
+#define ast2obj_constant ast2obj_object
 #define ast2obj_identifier ast2obj_object
 #define ast2obj_string ast2obj_object
 #define ast2obj_bytes ast2obj_object
@@ -746,6 +751,26 @@ static int obj2ast_object(PyObject* obj, PyObject** out, PyArena* arena)
     return 0;
 }
 
+static int obj2ast_constant(PyObject* obj, PyObject** out, PyArena* arena)
+{
+    if (obj == Py_None || obj == Py_True || obj == Py_False) {
+        /* don't increment the reference counter, Constant uses a borrowed
+         * reference, not a strong reference */
+        *out = obj;
+        return 0;
+    }
+
+    if (obj) {
+        if (PyArena_AddPyObject(arena, obj) < 0) {
+            *out = NULL;
+            return -1;
+        }
+        Py_INCREF(obj);
+    }
+    *out = obj;
+    return 0;
+}
+
 static int obj2ast_identifier(PyObject* obj, PyObject** out, PyArena* arena)
 {
     if (!PyUnicode_CheckExact(obj) && obj != Py_None) {
@@ -941,6 +966,8 @@ static int init_types(void)
     if (!NameConstant_type) return 0;
     Ellipsis_type = make_type("Ellipsis", expr_type, NULL, 0);
     if (!Ellipsis_type) return 0;
+    Constant_type = make_type("Constant", expr_type, Constant_fields, 1);
+    if (!Constant_type) return 0;
     Attribute_type = make_type("Attribute", expr_type, Attribute_fields, 3);
     if (!Attribute_type) return 0;
     Subscript_type = make_type("Subscript", expr_type, Subscript_fields, 3);
@@ -2167,6 +2194,25 @@ Ellipsis(int lineno, int col_offset, PyArena *arena)
 }
 
 expr_ty
+Constant(constant value, int lineno, int col_offset, PyArena *arena)
+{
+    expr_ty p;
+    if (!value) {
+        PyErr_SetString(PyExc_ValueError,
+                        "field value is required for Constant");
+        return NULL;
+    }
+    p = (expr_ty)PyArena_Malloc(arena, sizeof(*p));
+    if (!p)
+        return NULL;
+    p->kind = Constant_kind;
+    p->v.Constant.value = value;
+    p->lineno = lineno;
+    p->col_offset = col_offset;
+    return p;
+}
+
+expr_ty
 Attribute(expr_ty value, identifier attr, expr_context_ty ctx, int lineno, int
           col_offset, PyArena *arena)
 {
@@ -3267,6 +3313,15 @@ ast2obj_expr(void* _o)
         result = PyType_GenericNew(Ellipsis_type, NULL, NULL);
         if (!result) goto failed;
         break;
+    case Constant_kind:
+        result = PyType_GenericNew(Constant_type, NULL, NULL);
+        if (!result) goto failed;
+        value = ast2obj_constant(o->v.Constant.value);
+        if (!value) goto failed;
+        if (_PyObject_SetAttrId(result, &PyId_value, value) == -1)
+            goto failed;
+        Py_DECREF(value);
+        break;
     case Attribute_kind:
         result = PyType_GenericNew(Attribute_type, NULL, NULL);
         if (!result) goto failed;
@@ -6240,6 +6295,28 @@ obj2ast_expr(PyObject* obj, expr_ty* out, PyArena* arena)
         if (*out == NULL) goto failed;
         return 0;
     }
+    isinstance = PyObject_IsInstance(obj, (PyObject*)Constant_type);
+    if (isinstance == -1) {
+        return 1;
+    }
+    if (isinstance) {
+        constant value;
+
+        if (_PyObject_HasAttrId(obj, &PyId_value)) {
+            int res;
+            tmp = _PyObject_GetAttrId(obj, &PyId_value);
+            if (tmp == NULL) goto failed;
+            res = obj2ast_constant(tmp, &value, arena);
+            if (res != 0) goto failed;
+            Py_CLEAR(tmp);
+        } else {
+            PyErr_SetString(PyExc_TypeError, "required field \"value\" missing from Constant");
+            return 1;
+        }
+        *out = Constant(value, lineno, col_offset, arena);
+        if (*out == NULL) goto failed;
+        return 0;
+    }
     isinstance = PyObject_IsInstance(obj, (PyObject*)Attribute_type);
     if (isinstance == -1) {
         return 1;
@@ -7517,6 +7594,8 @@ PyInit__ast(void)
         0) return NULL;
     if (PyDict_SetItemString(d, "Ellipsis", (PyObject*)Ellipsis_type) < 0)
         return NULL;
+    if (PyDict_SetItemString(d, "Constant", (PyObject*)Constant_type) < 0)
+        return NULL;
     if (PyDict_SetItemString(d, "Attribute", (PyObject*)Attribute_type) < 0)
         return NULL;
     if (PyDict_SetItemString(d, "Subscript", (PyObject*)Subscript_type) < 0)
diff --git a/Python/ast.c b/Python/ast.c
index a5d8dba..5422e9c 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -132,6 +132,50 @@ validate_arguments(arguments_ty args)
 }
 
 static int
+validate_constant(PyObject *value)
+{
+    if (value == Py_None || value == Py_Ellipsis)
+        return 1;
+
+    if (PyLong_CheckExact(value)
+            || PyFloat_CheckExact(value)
+            || PyComplex_CheckExact(value)
+            || PyBool_Check(value)
+            || PyUnicode_CheckExact(value)
+            || PyBytes_CheckExact(value))
+        return 1;
+
+    if (PyTuple_CheckExact(value) || PyFrozenSet_CheckExact(value)) {
+        PyObject *it;
+
+        it = PyObject_GetIter(value);
+        if (it == NULL)
+            return 0;
+
+        while (1) {
+            PyObject *item = PyIter_Next(it);
+            if (item == NULL) {
+                if (PyErr_Occurred()) {
+                    Py_DECREF(it);
+                    return 0;
+                }
+                break;
+            }
+
+            if (!validate_constant(item)) {
+                Py_DECREF(it);
+                return 0;
+            }
+        }
+
+        Py_DECREF(it);
+        return 1;
+    }
+
+    return 0;
+}
+
+static int
 validate_expr(expr_ty exp, expr_context_ty ctx)
 {
     int check_ctx = 1;
@@ -240,6 +284,12 @@ validate_expr(expr_ty exp, expr_context_ty ctx)
         return validate_expr(exp->v.Call.func, Load) &&
             validate_exprs(exp->v.Call.args, Load, 0) &&
             validate_keywords(exp->v.Call.keywords);
+    case Constant_kind:
+        if (!validate_constant(exp->v.Constant.value)) {
+            PyErr_SetString(PyExc_TypeError, "invalid type in Constant");
+            return 0;
+        }
+        return 1;
     case Num_kind: {
         PyObject *n = exp->v.Num.n;
         if (!PyLong_CheckExact(n) && !PyFloat_CheckExact(n) &&
diff --git a/Python/compile.c b/Python/compile.c
index a710e82..ccb05cf 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -1314,7 +1314,11 @@ compiler_isdocstring(stmt_ty s)
 {
     if (s->kind != Expr_kind)
         return 0;
-    return s->v.Expr.value->kind == Str_kind;
+    if (s->v.Expr.value->kind == Str_kind)
+        return 1;
+    if (s->v.Expr.value->kind == Constant_kind)
+        return PyUnicode_CheckExact(s->v.Expr.value->v.Constant.value);
+    return 0;
 }
 
 /* Compile a sequence of statements, checking for a docstring. */
@@ -1688,8 +1692,12 @@ compiler_function(struct compiler *c, stmt_ty s, int is_async)
 
     st = (stmt_ty)asdl_seq_GET(body, 0);
     docstring = compiler_isdocstring(st);
-    if (docstring && c->c_optimize < 2)
-        first_const = st->v.Expr.value->v.Str.s;
+    if (docstring && c->c_optimize < 2) {
+        if (st->v.Expr.value->kind == Constant_kind)
+            first_const = st->v.Expr.value->v.Constant.value;
+        else
+            first_const = st->v.Expr.value->v.Str.s;
+    }
     if (compiler_add_o(c, c->u->u_consts, first_const) < 0)      {
         compiler_exit_scope(c);
         return 0;
@@ -2600,6 +2608,36 @@ compiler_assert(struct compiler *c, stmt_ty s)
 }
 
 static int
+compiler_visit_stmt_expr(struct compiler *c, expr_ty value)
+{
+    if (c->c_interactive && c->c_nestlevel <= 1) {
+        VISIT(c, expr, value);
+        ADDOP(c, PRINT_EXPR);
+        return 1;
+    }
+
+    if (value->kind == Str_kind || value->kind == Num_kind) {
+        /* ignore strings and numbers */
+        return 1;
+    }
+
+    if (value->kind == Constant_kind) {
+        PyObject *cst = value->v.Constant.value;
+        if (PyUnicode_CheckExact(cst)
+            || PyLong_CheckExact(cst)
+            || PyFloat_CheckExact(cst)
+            || PyComplex_CheckExact(cst)) {
+            /* ignore strings and numbers */
+            return 1;
+        }
+    }
+
+    VISIT(c, expr, value);
+    ADDOP(c, POP_TOP);
+    return 1;
+}
+
+static int
 compiler_visit_stmt(struct compiler *c, stmt_ty s)
 {
     Py_ssize_t i, n;
@@ -2669,16 +2707,7 @@ compiler_visit_stmt(struct compiler *c, stmt_ty s)
     case Nonlocal_kind:
         break;
     case Expr_kind:
-        if (c->c_interactive && c->c_nestlevel <= 1) {
-            VISIT(c, expr, s->v.Expr.value);
-            ADDOP(c, PRINT_EXPR);
-        }
-        else if (s->v.Expr.value->kind != Str_kind &&
-                 s->v.Expr.value->kind != Num_kind) {
-            VISIT(c, expr, s->v.Expr.value);
-            ADDOP(c, POP_TOP);
-        }
-        break;
+        return compiler_visit_stmt_expr(c, s->v.Expr.value);
     case Pass_kind:
         break;
     case Break_kind:
@@ -3625,6 +3654,8 @@ expr_constant(struct compiler *c, expr_ty e)
     switch (e->kind) {
     case Ellipsis_kind:
         return 1;
+    case Constant_kind:
+        return PyObject_IsTrue(e->v.Constant.value);
     case Num_kind:
         return PyObject_IsTrue(e->v.Num.n);
     case Str_kind:
@@ -3912,6 +3943,9 @@ compiler_visit_expr(struct compiler *c, expr_ty e)
         return compiler_compare(c, e);
     case Call_kind:
         return compiler_call(c, e);
+    case Constant_kind:
+        ADDOP_O(c, LOAD_CONST, e->v.Constant.value, consts);
+        break;
     case Num_kind:
         ADDOP_O(c, LOAD_CONST, e->v.Num.n, consts);
         break;
diff --git a/Python/future.c b/Python/future.c
index 163f87f..75f2107 100644
--- a/Python/future.c
+++ b/Python/future.c
@@ -79,7 +79,10 @@ future_parse(PyFutureFeatures *ff, mod_ty mod, PyObject *filename)
 
     i = 0;
     first = (stmt_ty)asdl_seq_GET(mod->v.Module.body, i);
-    if (first->kind == Expr_kind && first->v.Expr.value->kind == Str_kind)
+    if (first->kind == Expr_kind
+        && (first->v.Expr.value->kind == Str_kind
+            || (first->v.Expr.value->kind == Constant_kind
+                && PyUnicode_CheckExact(first->v.Expr.value->v.Constant.value))))
         i++;
 
 
diff --git a/Python/symtable.c b/Python/symtable.c
index 9f5149f..558bb06 100644
--- a/Python/symtable.c
+++ b/Python/symtable.c
@@ -1455,6 +1455,7 @@ symtable_visit_expr(struct symtable *st, expr_ty e)
     case JoinedStr_kind:
         VISIT_SEQ(st, expr, e->v.JoinedStr.values);
         break;
+    case Constant_kind:
     case Num_kind:
     case Str_kind:
     case Bytes_kind:
diff --git a/Tools/parser/unparse.py b/Tools/parser/unparse.py
index 9972797..35ebc66 100644
--- a/Tools/parser/unparse.py
+++ b/Tools/parser/unparse.py
@@ -343,6 +343,11 @@ class Unparser:
         value = t.s.replace("{", "{{").replace("}", "}}")
         write(value)
 
+    def _fstring_Constant(self, t, write):
+        assert isinstance(t.value, str)
+        value = t.value.replace("{", "{{").replace("}", "}}")
+        write(value)
+
     def _fstring_FormattedValue(self, t, write):
         write("{")
         expr = io.StringIO()
@@ -364,6 +369,25 @@ class Unparser:
     def _Name(self, t):
         self.write(t.id)
 
+    def _write_constant(self, value):
+        if isinstance(value, (float, complex)):
+            self.write(repr(value).replace("inf", INFSTR))
+        else:
+            self.write(repr(value))
+
+    def _Constant(self, t):
+        value = t.value
+        if isinstance(value, tuple):
+            self.write("(")
+            if len(value) == 1:
+                self._write_constant(value[0])
+                self.write(",")
+            else:
+                interleave(lambda: self.write(", "), self._write_constant, value)
+            self.write(")")
+        else:
+            self._write_constant(t.value)
+
     def _NameConstant(self, t):
         self.write(repr(t.value))
 
@@ -443,7 +467,7 @@ class Unparser:
     def _Tuple(self, t):
         self.write("(")
         if len(t.elts) == 1:
-            (elt,) = t.elts
+            elt = t.elts[0]
             self.dispatch(elt)
             self.write(",")
         else:
@@ -490,7 +514,8 @@ class Unparser:
         # Special case: 3.__abs__() is a syntax error, so if t.value
         # is an integer literal then we need to either parenthesize
         # it or add an extra space to get 3 .__abs__().
-        if isinstance(t.value, ast.Num) and isinstance(t.value.n, int):
+        if ((isinstance(t.value, ast.Num) and isinstance(t.value.n, int))
+           or (isinstance(t.value, ast.Constant) and isinstance(t.value.value, int))):
             self.write(" ")
         self.write(".")
         self.write(t.attr)