bpo-32892: Use ast.Constant instead of specific constant AST types. (GH-9445)

Author: serhiy-storchaka

Doc/library/ast.rst

@@ -78,18 +78,25 @@ Node classes
 
       node = ast.UnaryOp()
       node.op = ast.USub()
-      node.operand = ast.Num()
-      node.operand.n = 5
+      node.operand = ast.Constant()
+      node.operand.value = 5
       node.operand.lineno = 0
       node.operand.col_offset = 0
       node.lineno = 0
       node.col_offset = 0
 
    or the more compact ::
 
-      node = ast.UnaryOp(ast.USub(), ast.Num(5, lineno=0, col_offset=0),
+      node = ast.UnaryOp(ast.USub(), ast.Constant(5, lineno=0, col_offset=0),
                          lineno=0, col_offset=0)
 
+.. deprecated:: 3.8
+
+   Class :class:`ast.Constant` is now used for all constants. Old classes
+   :class:`ast.Num`, :class:`ast.Str`, :class:`ast.Bytes`,
+   :class:`ast.NameConstant` and :class:`ast.Ellipsis` are still available,
+   but they will be removed in future Python releases.
+
 
 .. _abstract-grammar:
 
@@ -239,7 +246,7 @@ and classes for traversing abstract syntax trees:
           def visit_Name(self, node):
               return copy_location(Subscript(
                   value=Name(id='data', ctx=Load()),
-                  slice=Index(value=Str(s=node.id)),
+                  slice=Index(value=Constant(value=node.id)),
                   ctx=node.ctx
               ), node)
 

Doc/whatsnew/3.8.rst

@@ -262,6 +262,11 @@ Deprecated
 
   (Contributed by Berker Peksag in :issue:`9372`.)
 
+* :mod:`ast` classes ``Num``, ``Str``, ``Bytes``, ``NameConstant`` and
+  ``Ellipsis`` are considered deprecated and will be removed in future Python
+  versions. :class:`~ast.Constant` should be used instead.
+  (Contributed by Serhiy Storchaka in :issue:`32892`.)
+
 
 Removed
 =======

Include/Python-ast.h

@@ -48,10 +48,8 @@ def literal_eval(node_or_string):
         node_or_string = node_or_string.body
     def _convert_num(node):
         if isinstance(node, Constant):
-            if isinstance(node.value, (int, float, complex)):
+            if type(node.value) in (int, float, complex):
                 return node.value
-        elif isinstance(node, Num):
-            return node.n
         raise ValueError('malformed node or string: ' + repr(node))
     def _convert_signed_num(node):
         if isinstance(node, UnaryOp) and isinstance(node.op, (UAdd, USub)):
@@ -64,10 +62,6 @@ def _convert_signed_num(node):
     def _convert(node):
         if isinstance(node, Constant):
             return node.value
-        elif isinstance(node, (Str, Bytes)):
-            return node.s
-        elif isinstance(node, Num):
-            return node.n
         elif isinstance(node, Tuple):
             return tuple(map(_convert, node.elts))
         elif isinstance(node, List):
@@ -77,8 +71,6 @@ def _convert(node):
         elif isinstance(node, Dict):
             return dict(zip(map(_convert, node.keys),
                             map(_convert, node.values)))
-        elif isinstance(node, NameConstant):
-            return node.value
         elif isinstance(node, BinOp) and isinstance(node.op, (Add, Sub)):
             left = _convert_signed_num(node.left)
             right = _convert_num(node.right)
@@ -329,3 +321,66 @@ def generic_visit(self, node):
                 else:
                     setattr(node, field, new_node)
         return node
+
+
+# The following code is for backward compatibility.
+# It will be removed in future.
+
+def _getter(self):
+    return self.value
+
+def _setter(self, value):
+    self.value = value
+
+Constant.n = property(_getter, _setter)
+Constant.s = property(_getter, _setter)
+
+class _ABC(type):
+
+    def __instancecheck__(cls, inst):
+        if not isinstance(inst, Constant):
+            return False
+        if cls in _const_types:
+            try:
+                value = inst.value
+            except AttributeError:
+                return False
+            else:
+                return type(value) in _const_types[cls]
+        return type.__instancecheck__(cls, inst)
+
+def _new(cls, *args, **kwargs):
+    if cls in _const_types:
+        return Constant(*args, **kwargs)
+    return Constant.__new__(cls, *args, **kwargs)
+
+class Num(Constant, metaclass=_ABC):
+    _fields = ('n',)
+    __new__ = _new
+
+class Str(Constant, metaclass=_ABC):
+    _fields = ('s',)
+    __new__ = _new
+
+class Bytes(Constant, metaclass=_ABC):
+    _fields = ('s',)
+    __new__ = _new
+
+class NameConstant(Constant, metaclass=_ABC):
+    __new__ = _new
+
+class Ellipsis(Constant, metaclass=_ABC):
+    _fields = ()
+
+    def __new__(cls, *args, **kwargs):
+        if cls is Ellipsis:
+            return Constant(..., *args, **kwargs)
+        return Constant.__new__(cls, *args, **kwargs)
+
+_const_types = {
+    Num: (int, float, complex),
+    Str: (str,),
+    Bytes: (bytes,),
+    NameConstant: (type(None), bool),
+    Ellipsis: (type(...),),
+}

Lib/ast.py

@@ -2005,14 +2005,8 @@ def wrap_value(s):
             except NameError:
                 raise RuntimeError()
 
-        if isinstance(value, str):
-            return ast.Str(value)
-        if isinstance(value, (int, float)):
-            return ast.Num(value)
-        if isinstance(value, bytes):
-            return ast.Bytes(value)
-        if value in (True, False, None):
-            return ast.NameConstant(value)
+        if isinstance(value, (str, int, float, bytes, bool, type(None))):
+            return ast.Constant(value)
         raise RuntimeError()
 
     class RewriteSymbolics(ast.NodeTransformer):