the Slice in x[::] has to have step as None to help the interpreter

diff --git a/Lib/test/test_ast.py b/Lib/test/test_ast.py
index 4e311e44730b63cb436161bc34d3fea0aae52920..6dca5d2e65e6e4ec59dac7e6d9dc47528fdeb7f2 100644 (file)
--- a/Lib/test/test_ast.py
+++ b/Lib/test/test_ast.py
@@ -150,7 +150,8 @@ class AST_Tests(unittest.TestCase):
         slc = ast.parse("x[::]").body[0].value.slice
         self.assertIsNone(slc.upper)
         self.assertIsNone(slc.lower)
-        self.assertIsNone(slc.step)
+        self.assertTrue(isinstance(slc.step, ast.Name))
+        self.assertEqual(slc.step.id, "None")
 
     def test_from_import(self):
         im = ast.parse("from . import y").body[0]

diff --git a/Misc/NEWS b/Misc/NEWS
index 82c07b4e50cae2c01f91509ba8499becb300a99d..77e3ac6acae56258ac874d6d6d77d5b4910c258b 100644 (file)
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -44,9 +44,6 @@ Core and Builtins
 
 - Assignment to None using import statements now raises a SyntaxError.
 
-- In the slice AST type, the step field will always be None if a step expression
-  is not specified.
-
 - Issue #4547: When debugging a very large function, it was not always
   possible to update the lineno attribute of the current frame.
 

diff --git a/Python/ast.c b/Python/ast.c
index 6ccd02fc704713f4e0e77cd456744070ced6755e..0644da830052cfaebafebf969f3c7e078644d19c 100644 (file)
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -1471,7 +1471,21 @@ ast_for_slice(struct compiling *c, const node *n)
 
     ch = CHILD(n, NCH(n) - 1);
     if (TYPE(ch) == sliceop) {
-        if (NCH(ch) != 1) {
+        if (NCH(ch) == 1) {
+            /* 
+              This is an extended slice (ie "x[::]") with no expression in the
+              step field. We set this literally to "None" in order to
+              disambiguate it from x[:]. (The interpreter might have to call
+              __getslice__ for x[:], but it must call __getitem__ for x[::].)
+            */
+            identifier none = new_identifier("None", c->c_arena);
+            if (!none)
+                return NULL;
+            ch = CHILD(ch, 0);
+            step = Name(none, Load, LINENO(ch), ch->n_col_offset, c->c_arena);
+            if (!step)
+                return NULL;
+        } else {
             ch = CHILD(ch, 1);
             if (TYPE(ch) == test) {
                 step = ast_for_expr(c, ch);