#4617: Previously it was illegal to delete a name from the local

namespace if it occurs as a free variable in a nested block.  This limitation
of the compiler has been lifted, and a new opcode introduced (DELETE_DEREF).

This sample was valid in 2.6, but fails to compile in 3.x without this change::

   >>> def f():
   ...     def print_error():
   ...        print(e)
   ...     try:
   ...        something
   ...     except Exception as e:
   ...        print_error()
   ...        # implicit "del e" here


This sample has always been invalid in Python, and now works::

   >>> def outer(x):
   ...     def inner():
   ...        return x
   ...     inner()
   ...     del x

There is no need to bump the PYC magic number: the new opcode is used
for code that did not compile before.

1 files changed, 3 insertions, 7 deletions

diff --git a/Python/compile.c b/Python/compile.c
index 6963a151a7..5341e6b535 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -857,6 +857,8 @@ opcode_stack_effect(int opcode, int oparg)
             return 1;
         case STORE_DEREF:
             return -1;
+        case DELETE_DEREF:
+            return 0;
         default:
             fprintf(stderr, "opcode = %d\n", opcode);
             Py_FatalError("opcode_stack_effect()");
@@ -2506,13 +2508,7 @@ compiler_nameop(struct compiler *c, identifier name, expr_context_ty ctx)
         case AugLoad:
         case AugStore:
             break;
-        case Del:
-            PyErr_Format(PyExc_SyntaxError,
-                         "can not delete variable '%S' referenced "
-                         "in nested scope",
-                         name);
-            Py_DECREF(mangled);
-            return 0;
+        case Del: op = DELETE_DEREF; break;
         case Param:
         default:
             PyErr_SetString(PyExc_SystemError,