From e9ae8a56947593860a4b23d1442d77700232909c Mon Sep 17 00:00:00 2001
From: Barry Warsaw <barry@python.org>
Date: Sat, 2 Dec 2000 04:11:32 +0000
Subject: [PATCH] Talk about PEP 213, which is related and can be (mostly)
 implemented using the __findattr__() hook.

---
 pep-0231.txt | 69 +++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 66 insertions(+), 3 deletions(-)

diff --git a/pep-0231.txt b/pep-0231.txt
index cf81ac773..99ecdf5bd 100644
--- a/pep-0231.txt
+++ b/pep-0231.txt
@@ -110,6 +110,16 @@ Key Differences with the Existing Protocol
     is used.
 
 
+Related Work
+
+    PEP 213 [9] describes a different approach to hooking into
+    attribute access and modification.  The semantics proposed in PEP
+    213 can be implemented using the __findattr__() hook described
+    here, with one caveat.  The current reference implementation of
+    __findattr__() does not support hooking on attribute deletion.
+    This could be added if it's found desirable.  See example below.
+
+
 Examples
 
     One programming style that this proposal allows is a Java
@@ -368,9 +378,9 @@ Examples
             assert 0, 'AttributeError expected'
     
 
-    And finally, C++-like access control can also be accomplished,
-    although less cleanly because of the difficulty of figuring out
-    what method is being called from the runtime call stack:
+    C++-like access control can also be accomplished, although less
+    cleanly because of the difficulty of figuring out what method is
+    being called from the runtime call stack:
 
         import sys
         import types
@@ -520,6 +530,58 @@ Examples
         assert b.getname() == 'A'
 
 
+    Here's an implementation of the attribute hook described in PEP
+    213 (except that hooking on attribute deletion isn't supported by
+    the current reference implementation).
+
+        class Pep213:
+            def __findattr__(self, name, *args):
+                hookname = '__attr_%s__' % name
+                if args:
+                    op = 'set'
+                else:
+                    op = 'get'
+                # XXX: op = 'del' currently not supported
+                missing = ()
+                meth = getattr(self, hookname, missing)
+                if meth is missing:
+                    if op == 'set':
+                        return setattr(self, name, *args)
+                    else:
+                        return getattr(self, name)
+                else:
+                    return meth(op, *args)
+
+
+        def computation(i):
+            print 'doing computation:', i
+            return i + 3
+
+
+        def rev_computation(i):
+            print 'doing rev_computation:', i
+            return i - 3
+
+
+        class X(Pep213):
+            def __init__(self, foo=0):
+                self.__foo = foo
+
+            def __attr_foo__(self, op, val=None):
+                if op == 'get':
+                    return computation(self.__foo)
+                elif op == 'set':
+                    self.__foo = rev_computation(val)
+                # XXX: 'del' not yet supported
+
+        x = X()
+        fooval = x.foo
+        print fooval
+        x.foo = fooval + 5
+        print x.foo
+        # del x.foo
+
+
 Reference Implementation
 
    The reference implementation, as a patch to the Python core, can be
@@ -537,6 +599,7 @@ References
     [6] http://www.python.org/doc/essays/metaclasses/
     [7] http://www.foretec.com/python/workshops/1998-11/dd-ascher-sum.html
     [8] http://www.python.org/doc/howto/rexec/rexec.html
+    [9] http://python.sourceforge.net/peps/pep-0213.html
 
 
 Copyright