Reflect AST changes in Python 3.8.

* Num, Str, Bytes, Ellipsis and NameConstant are replaced with Constant.
  (https://bugs.python.org/issue32892)

* Index is replaced with its value, ExtSlice is replaced with Tuple.
  (https://bugs.python.org/issue34822)

Author: serhiy-storchaka

astroid/as_string.py

@@ -188,6 +188,8 @@ def visit_comprehension(self, node):
 
     def visit_const(self, node):
         """return an astroid.Const node as string"""
+        if node.value is Ellipsis:
+            return '...'
         return repr(node.value)
 
     def visit_continue(self, node):
@@ -273,7 +275,7 @@ def visit_exec(self, node):
 
     def visit_extslice(self, node):
         """return an astroid.ExtSlice node as string"""
-        return ','.join([dim.accept(self) for dim in node.dims])
+        return ', '.join([dim.accept(self) for dim in node.dims])
 
     def visit_for(self, node):
         """return an astroid.For node as string"""
@@ -430,8 +432,12 @@ def visit_slice(self, node):
 
     def visit_subscript(self, node):
         """return an astroid.Subscript node as string"""
-        return '%s[%s]' % (self._precedence_parens(node, node.value),
-                           node.slice.accept(self))
+        idx = node.slice.accept(self)
+        if node.slice.__class__.__name__.lower() == 'tuple' and node.slice.elts:
+            # Remove parenthesis in tuple and extended slice.
+            # a[(::1, 1:)] is not valid syntax.
+            idx = idx[1:-1]
+        return '%s[%s]' % (self._precedence_parens(node, node.value), idx)
 
     def visit_tryexcept(self, node):
         """return an astroid.TryExcept node as string"""

astroid/rebuilder.py

@@ -479,6 +479,7 @@ def visit_expr(self, node, parent):
         newnode.postinit(self.visit(node.value, newnode))
         return newnode
 
+    # Not used in Python 3.8+.
     def visit_ellipsis(self, node, parent):
         """visit an Ellipsis node by returning a fresh instance of it"""
         return nodes.Ellipsis(getattr(node, 'lineno', None),
@@ -509,6 +510,7 @@ def visit_exec(self, node, parent):
                          _visit_or_none(node, 'locals', self, newnode))
         return newnode
 
+    # Not used in Python 3.8+.
     def visit_extslice(self, node, parent):
         """visit an ExtSlice node by returning a fresh instance of it"""
         newnode = nodes.ExtSlice(parent=parent)
@@ -641,6 +643,7 @@ def visit_import(self, node, parent):
             parent.set_local(name.split('.')[0], newnode)
         return newnode
 
+    # Not used in Python 3.8+.
     def visit_index(self, node, parent):
         """visit a Index node by returning a fresh instance of it"""
         newnode = nodes.Index(parent=parent)
@@ -702,12 +705,19 @@ def visit_name(self, node, parent):
             self._save_assignment(newnode)
         return newnode
 
+    def visit_constant(self, node, parent):
+        """visit a Constant node by returning a fresh instance of Const"""
+        return nodes.Const(node.value, getattr(node, 'lineno', None),
+                           getattr(node, 'col_offset', None), parent)
+
+    # Not used in Python 3.8+.
     def visit_str(self, node, parent):
         """visit a String/Bytes node by returning a fresh instance of Const"""
         return nodes.Const(node.s, getattr(node, 'lineno', None),
                            getattr(node, 'col_offset', None), parent)
     visit_bytes = visit_str
 
+    # Not used in Python 3.8+.
     def visit_num(self, node, parent):
         """visit a Num node by returning a fresh instance of Const"""
         return nodes.Const(node.n, getattr(node, 'lineno', None),
@@ -854,6 +864,7 @@ def visit_arg(self, node, parent):
         """visit an arg node by returning a fresh AssName instance"""
         return self.visit_assignname(node, parent, node.arg)
 
+    # Not used in Python 3.8+.
     def visit_nameconstant(self, node, parent):
         # in Python 3.4 we have NameConstant for True / False / None
         return nodes.Const(node.value, getattr(node, 'lineno', None),

astroid/tests/unittest_nodes.py

@@ -158,8 +158,8 @@ def test_ellipsis(self):
         self.assertEqual(ast.as_string(), 'a[...]')
 
     def test_slices(self):
-        for code in ('a[0]', 'a[1:3]', 'a[:-1:step]', 'a[:,newaxis]',
-                     'a[newaxis,:]', 'del L[::2]', 'del A[1]', 'del Br[:]'):
+        for code in ('a[0]', 'a[1:3]', 'a[:-1:step]', 'a[:, newaxis]',
+                     'a[newaxis, :]', 'del L[::2]', 'del A[1]', 'del Br[:]'):
             ast = abuilder.string_build(code).body[0]
             self.assertEqual(ast.as_string(), code)
 
@@ -170,7 +170,7 @@ def test_slice_and_subscripts(self):
 bord[2:]
 del av[d::f], a[df:]
 a[:1] = bord[2:]
-del SRC[::1,newaxis,1:]
+del SRC[::1, newaxis, 1:]
 tous[vals] = 1010
 del thousand[key]
 del a[::2], a[:-1:step]
@@ -546,7 +546,7 @@ def test_as_string(self):
             print()
             test: int = 5
             test2: str
-            test3: List[Dict[(str, str)]] = []
+            test3: List[Dict[str, str]] = []
         """)
         ast = abuilder.string_build(code)
         self.assertEqual(ast.as_string().strip(), code.strip())