#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.

3 files changed, 35 insertions, 5 deletions

diff --git a/Doc/library/dis.rst b/Doc/library/dis.rst
index fb62ec1..8e57dd4 100644
--- a/Doc/library/dis.rst
+++ b/Doc/library/dis.rst
@@ -723,6 +723,12 @@ the more significant byte last.
    storage.
 
 
+.. opcode:: DELETE_DEREF (i)
+
+   Empties the cell contained in slot *i* of the cell and free variable storage.
+   Used by the :keyword:`del` statement.
+
+
 .. opcode:: SET_LINENO (lineno)
 
    This opcode is obsolete.
diff --git a/Doc/reference/simple_stmts.rst b/Doc/reference/simple_stmts.rst
index 378efc7..4082aa7 100644
--- a/Doc/reference/simple_stmts.rst
+++ b/Doc/reference/simple_stmts.rst
@@ -388,11 +388,6 @@ namespace, depending on whether the name occurs in a :keyword:`global` statement
 in the same code block.  If the name is unbound, a :exc:`NameError` exception
 will be raised.
 
-.. index:: pair: free; variable
-
-It is illegal to delete a name from the local namespace if it occurs as a free
-variable in a nested block.
-
 .. index:: pair: attribute; deletion
 
 Deletion of attribute references, subscriptions and slicings is passed to the
@@ -400,6 +395,10 @@ primary object involved; deletion of a slicing is in general equivalent to
 assignment of an empty slice of the right type (but even this is determined by
 the sliced object).
 
+.. versionchanged:: 3.2
+   Previously it was illegal to delete a name from the local namespace if it
+   occurs as a free variable in a nested block.
+
 
 .. _return:
 
diff --git a/Doc/whatsnew/3.2.rst b/Doc/whatsnew/3.2.rst
index 7d8970b..bd38a7e 100644
--- a/Doc/whatsnew/3.2.rst
+++ b/Doc/whatsnew/3.2.rst
@@ -240,6 +240,31 @@ Some smaller changes made to the core Python language are:
 
   (See :issue:`8188`.)
 
+* Previously it was illegal to delete a name from the local namespace if it
+  occurs as a free variable in a nested block::
+
+   >>> def outer(x):
+   ...     def inner():
+   ...        return x
+   ...     inner()
+   ...     del x
+
+  This is now allowed.  Remember that the target of an :keyword:`except` clause
+  is cleared, so this code which used to work with Python 2.6, raised a
+  :exc:`SyntaxError` with Python 3.1 and now works again::
+
+   >>> def f():
+   ...     def print_error():
+   ...        print(e)
+   ...     try:
+   ...        something
+   ...     except Exception as e:
+   ...        print_error()
+   ...        # implicit "del e" here
+
+  (See :issue:`4617`.)
+
+
 New, Improved, and Deprecated Modules
 =====================================