#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.

4 files changed, 35 insertions, 16 deletions

diff --git a/Lib/opcode.py b/Lib/opcode.py
index 9833b6c..8e15d13 100644
--- a/Lib/opcode.py
+++ b/Lib/opcode.py
@@ -161,6 +161,8 @@ def_op('LOAD_DEREF', 136)
 hasfree.append(136)
 def_op('STORE_DEREF', 137)
 hasfree.append(137)
+def_op('DELETE_DEREF', 138)
+hasfree.append(138)
 
 def_op('CALL_FUNCTION_VAR', 140)     # #args + (#kwargs << 8)
 def_op('CALL_FUNCTION_KW', 141)      # #args + (#kwargs << 8)
diff --git a/Lib/test/test_exceptions.py b/Lib/test/test_exceptions.py
index 126a57f..e2824b2 100644
--- a/Lib/test/test_exceptions.py
+++ b/Lib/test/test_exceptions.py
@@ -526,6 +526,17 @@ class ExceptionTests(unittest.TestCase):
         obj = wr()
         self.assertTrue(obj is None, "%s" % obj)
 
+    def test_exception_target_in_nested_scope(self):
+        # issue 4617: This used to raise a SyntaxError
+        # "can not delete variable 'e' referenced in nested scope"
+        def print_error():
+            e
+        try:
+            something
+        except Exception as e:
+            print_error()
+            # implicit "del e" here
+
     def test_generator_leaking(self):
         # Test that generator exception state doesn't leak into the calling
         # frame
diff --git a/Lib/test/test_scope.py b/Lib/test/test_scope.py
index 2ac34cf..0e5a8e9 100644
--- a/Lib/test/test_scope.py
+++ b/Lib/test/test_scope.py
@@ -218,13 +218,6 @@ class ScopeTests(unittest.TestCase):
             """)
 
         check_syntax_error(self, """if 1:
-            def f(x):
-                def g():
-                    return x
-                del x # can't del name
-            """)
-
-        check_syntax_error(self, """if 1:
             def f():
                 def g():
                     from sys import *
@@ -272,6 +265,28 @@ class ScopeTests(unittest.TestCase):
         self.assertRaises(UnboundLocalError, errorInOuter)
         self.assertRaises(NameError, errorInInner)
 
+    def testUnboundLocal_AfterDel(self):
+        # #4617: It is now legal to delete a cell variable.
+        # The following functions must obviously compile,
+        # and give the correct error when accessing the deleted name.
+        def errorInOuter():
+            y = 1
+            del y
+            print(y)
+            def inner():
+                return y
+
+        def errorInInner():
+            def inner():
+                return y
+            y = 1
+            del y
+            inner()
+
+        self.assertRaises(UnboundLocalError, errorInOuter)
+        self.assertRaises(NameError, errorInInner)
+
+    def testUnboundLocal_AugAssign(self):
         # test for bug #1501934: incorrect LOAD/STORE_GLOBAL generation
         exec("""if 1:
             global_x = 1
diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index f354311..cd6b9a5 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -564,15 +564,6 @@ class SyntaxTestCase(unittest.TestCase):
     def test_break_outside_loop(self):
         self._check_error("break", "outside loop")
 
-    def test_delete_deref(self):
-        source = """if 1:
-            def foo(x):
-              def bar():
-                print(x)
-              del x
-            """
-        self._check_error(source, "nested scope")
-
     def test_unexpected_indent(self):
         self._check_error("foo()\n bar()\n", "unexpected indent",
                           subclass=IndentationError)