bpo-34822: Simplify AST for subscription.

Doc/library/ast.rst

@@ -90,12 +90,19 @@ Node classes
       node = ast.UnaryOp(ast.USub(), ast.Constant(5, lineno=0, col_offset=0),
                          lineno=0, col_offset=0)
 
+.. versionchanged:: 3.8
+
+   Class :class:`ast.Constant` is now used for all constants.
+   Simple indices are represented by their value, extended slices are
+   represented as tuples.
+
 .. 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.
+   Old classes :class:`ast.Num`, :class:`ast.Str`, :class:`ast.Bytes`,
+   :class:`ast.NameConstant`, :class:`ast.Ellipsis`, :class:`ast.Index`
+   and :class:`ast.ExtSlice` are still available, but they will be removed
+   in future Python releases. In meanwhile, instantiating them will return
+   an instance of different class.
 
 
 .. _abstract-grammar:
@@ -246,7 +253,7 @@ 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=Constant(value=node.id)),
+                  slice=Constant(value=node.id),
                   ctx=node.ctx
               ), node)
 

Doc/whatsnew/3.8.rst

@@ -267,6 +267,12 @@ Deprecated
   versions. :class:`~ast.Constant` should be used instead.
   (Contributed by Serhiy Storchaka in :issue:`32892`.)
 
+* :mod:`ast` classes ``Index`` and ``ExtSlice`` are considered deprecated
+  and will be removed in future Python versions.  Just ``value`` should be
+  used instead of ``Index(value)``.  ``Tuple(slices, Load())`` should be
+  used instead of ``ExtSlice(slices)``.
+  (Contributed by Serhiy Storchaka in :issue:`32892`.)
+
 
 Removed
 =======
@@ -370,6 +376,19 @@ Changes in the Python API
   external entities by default.
   (Contributed by Christian Heimes in :issue:`17239`.)
 
+* Simplified AST for literals and subscription.
+
+  - All constants will be represented as :class:`ast.Constant` instances.
+    Instantiating old classes ``Num``, ``Str``, ``Bytes``, ``NameConstant``
+    and ``Ellipsis`` will return an instance of ``Constant``.
+    (Contributed by Serhiy Storchaka in :issue:`32892`.)
+
+  - Simple indices will be represented by their value, extended slices will
+    be represented as tuples.  ``Index(value)`` will return a ``value`` itself,
+    ``ExtSlice(slices)`` will return ``Tuple(slices, Load())``.
+    (Contributed by Serhiy Storchaka in :issue:`34822`.)
+
+
 CPython bytecode changes
 ------------------------
 

Lib/ast.py

@@ -384,3 +384,11 @@ def __new__(cls, *args, **kwargs):
     NameConstant: (type(None), bool),
     Ellipsis: (type(...),),
 }
+
+class Index(AST):
+    def __new__(cls, value, *args, **kwargs):
+        return value
+
+class ExtSlice(AST):
+    def __new__(cls, dims=(), *args, **kwargs):
+        return Tuple(list(dims), Load(), *args, **kwargs)

Lib/test/test_ast.py

@@ -277,6 +277,10 @@ def test_base_classes(self):
     def test_field_attr_existence(self):
         for name, item in ast.__dict__.items():
             if isinstance(item, type) and name != 'AST' and name[0].isupper():
+                if name == 'Index':
+                    # Index(value) just returns value now.
+                    # The argument is required.
+                    continue
                 x = item()
                 if isinstance(x, ast.AST):
                     self.assertEqual(type(x._fields), tuple)
@@ -1056,21 +1060,21 @@ def test_attribute(self):
         self.expr(attr, "must have Load context")
 
     def test_subscript(self):
-        sub = ast.Subscript(ast.Name("x", ast.Store()), ast.Index(ast.Num(3)),
+        sub = ast.Subscript(ast.Name("x", ast.Store()), ast.Num(3),
                             ast.Load())
         self.expr(sub, "must have Load context")
         x = ast.Name("x", ast.Load())
-        sub = ast.Subscript(x, ast.Index(ast.Name("y", ast.Store())),
+        sub = ast.Subscript(x, ast.Name("y", ast.Store()),
                             ast.Load())
         self.expr(sub, "must have Load context")
         s = ast.Name("x", ast.Store())
         for args in (s, None, None), (None, s, None), (None, None, s):
             sl = ast.Slice(*args)
             self.expr(ast.Subscript(x, sl, ast.Load()),
                       "must have Load context")
-        sl = ast.ExtSlice([])
-        self.expr(ast.Subscript(x, sl, ast.Load()), "empty dims on ExtSlice")
-        sl = ast.ExtSlice([ast.Index(s)])
+        sl = ast.Tuple([], ast.Load())
+        self.expr(ast.Subscript(x, sl, ast.Load()))
+        sl = ast.Tuple([s], ast.Load())
         self.expr(ast.Subscript(x, sl, ast.Load()), "must have Load context")
 
     def test_starred(self):
@@ -1306,13 +1310,13 @@ def main():
 ('Expression', ('Constant', (1, 0), 10)),
 ('Expression', ('Constant', (1, 0), 'string')),
 ('Expression', ('Attribute', (1, 0), ('Name', (1, 0), 'a', ('Load',)), 'b', ('Load',))),
-('Expression', ('Subscript', (1, 0), ('Name', (1, 0), 'a', ('Load',)), ('Slice', ('Name', (1, 2), 'b', ('Load',)), ('Name', (1, 4), 'c', ('Load',)), None), ('Load',))),
+('Expression', ('Subscript', (1, 0), ('Name', (1, 0), 'a', ('Load',)), ('Slice', (1, 2), ('Name', (1, 2), 'b', ('Load',)), ('Name', (1, 4), 'c', ('Load',)), None), ('Load',))),
 ('Expression', ('Name', (1, 0), 'v', ('Load',))),
 ('Expression', ('List', (1, 0), [('Constant', (1, 1), 1), ('Constant', (1, 3), 2), ('Constant', (1, 5), 3)], ('Load',))),
 ('Expression', ('List', (1, 0), [], ('Load',))),
 ('Expression', ('Tuple', (1, 0), [('Constant', (1, 0), 1), ('Constant', (1, 2), 2), ('Constant', (1, 4), 3)], ('Load',))),
 ('Expression', ('Tuple', (1, 1), [('Constant', (1, 1), 1), ('Constant', (1, 3), 2), ('Constant', (1, 5), 3)], ('Load',))),
 ('Expression', ('Tuple', (1, 0), [], ('Load',))),
-('Expression', ('Call', (1, 0), ('Attribute', (1, 0), ('Attribute', (1, 0), ('Attribute', (1, 0), ('Name', (1, 0), 'a', ('Load',)), 'b', ('Load',)), 'c', ('Load',)), 'd', ('Load',)), [('Subscript', (1, 8), ('Attribute', (1, 8), ('Name', (1, 8), 'a', ('Load',)), 'b', ('Load',)), ('Slice', ('Constant', (1, 12), 1), ('Constant', (1, 14), 2), None), ('Load',))], [])),
+('Expression', ('Call', (1, 0), ('Attribute', (1, 0), ('Attribute', (1, 0), ('Attribute', (1, 0), ('Name', (1, 0), 'a', ('Load',)), 'b', ('Load',)), 'c', ('Load',)), 'd', ('Load',)), [('Subscript', (1, 8), ('Attribute', (1, 8), ('Name', (1, 8), 'a', ('Load',)), 'b', ('Load',)), ('Slice', (1, 12), ('Constant', (1, 12), 1), ('Constant', (1, 14), 2), None), ('Load',))], [])),
 ]
 main()

Misc/NEWS.d/next/Library/2018-09-27-19-31-47.bpo-34822.EztBhL.rst

@@ -0,0 +1,5 @@
+Simplified AST for subscription. Simple indices will be represented by their
+value, extended slices will be represented as tuples.  :mod:`ast` classes
+``Index`` and ``ExtSlice`` are considered deprecated and will be removed in
+future Python versions. In meanwhile, ``Index(value)`` will return a ``value``
+itself, ``ExtSlice(slices)`` will return ``Tuple(slices, Load())``.

Parser/Python.asdl

@@ -78,21 +78,20 @@ module Python
 
          -- the following expression can appear in assignment context
          | Attribute(expr value, identifier attr, expr_context ctx)
-         | Subscript(expr value, slice slice, expr_context ctx)
+         | Subscript(expr value, expr slice, expr_context ctx)
          | Starred(expr value, expr_context ctx)
          | Name(identifier id, expr_context ctx)
          | List(expr* elts, expr_context ctx)
          | Tuple(expr* elts, expr_context ctx)
 
+         -- can appear only in Subscript
+         | Slice(expr? lower, expr? upper, expr? step)
+
           -- col_offset is the byte offset in the utf8 string the parser uses
           attributes (int lineno, int col_offset)
 
     expr_context = Load | Store | Del | AugLoad | AugStore | Param
 
-    slice = Slice(expr? lower, expr? upper, expr? step)
-          | ExtSlice(slice* dims)
-          | Index(expr value)
-
     boolop = And | Or
 
     operator = Add | Sub | Mult | MatMult | Div | Mod | Pow | LShift