#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.

Python/ceval.c

@@ -135,6 +135,7 @@ static PyObject * cmp_outcome(int, PyObject *, PyObject *);
 static PyObject * import_from(PyObject *, PyObject *);
 static int import_all_from(PyObject *, PyObject *);
 static void format_exc_check_arg(PyObject *, const char *, PyObject *);
+static void format_exc_unbound(PyCodeObject *co, int oparg);
 static PyObject * unicode_concatenate(PyObject *, PyObject *,
                                       PyFrameObject *, unsigned char *);
 static PyObject * special_lookup(PyObject *, char *, PyObject **);
@@ -2143,6 +2144,16 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
                 );
             break;
 
+        TARGET(DELETE_DEREF)
+            x = freevars[oparg];
+            if (PyCell_GET(x) != NULL) {
+                PyCell_Set(x, NULL);
+                continue;
+            }
+            err = -1;
+            format_exc_unbound(co, oparg);
+            break;
+
         TARGET(LOAD_CLOSURE)
             x = freevars[oparg];
             Py_INCREF(x);
@@ -2158,22 +2169,7 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
                 DISPATCH();
             }
             err = -1;
-            /* Don't stomp existing exception */
-            if (PyErr_Occurred())
-                break;
-            if (oparg < PyTuple_GET_SIZE(co->co_cellvars)) {
-                v = PyTuple_GET_ITEM(co->co_cellvars,
-                                     oparg);
-                format_exc_check_arg(
-                       PyExc_UnboundLocalError,
-                       UNBOUNDLOCAL_ERROR_MSG,
-                       v);
-            } else {
-                v = PyTuple_GET_ITEM(co->co_freevars, oparg -
-                    PyTuple_GET_SIZE(co->co_cellvars));
-                format_exc_check_arg(PyExc_NameError,
-                                     UNBOUNDFREE_ERROR_MSG, v);
-            }
+            format_exc_unbound(co, oparg);
             break;
 
         TARGET(STORE_DEREF)
@@ -4352,6 +4348,28 @@ format_exc_check_arg(PyObject *exc, const char *format_str, PyObject *obj)
     PyErr_Format(exc, format_str, obj_str);
 }
 
+static void
+format_exc_unbound(PyCodeObject *co, int oparg)
+{
+    PyObject *name;
+    /* Don't stomp existing exception */
+    if (PyErr_Occurred())
+        return;
+    if (oparg < PyTuple_GET_SIZE(co->co_cellvars)) {
+        name = PyTuple_GET_ITEM(co->co_cellvars,
+                                oparg);
+        format_exc_check_arg(
+            PyExc_UnboundLocalError,
+            UNBOUNDLOCAL_ERROR_MSG,
+            name);
+    } else {
+        name = PyTuple_GET_ITEM(co->co_freevars, oparg -
+                                PyTuple_GET_SIZE(co->co_cellvars));
+        format_exc_check_arg(PyExc_NameError,
+                             UNBOUNDFREE_ERROR_MSG, name);
+    }
+}
+
 static PyObject *
 unicode_concatenate(PyObject *v, PyObject *w,
                    PyFrameObject *f, unsigned char *next_instr)

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,

Python/opcode_targets.h

@@ -137,7 +137,7 @@ static void *opcode_targets[256] = {
     &&TARGET_LOAD_CLOSURE,
     &&TARGET_LOAD_DEREF,
     &&TARGET_STORE_DEREF,
-    &&_unknown_opcode,
+    &&TARGET_DELETE_DEREF,
     &&_unknown_opcode,
     &&TARGET_CALL_FUNCTION_VAR,
     &&TARGET_CALL_FUNCTION_KW,