From 7b877a9278b5e8a3dcac5d3a485ed657ac8815fe Mon Sep 17 00:00:00 2001
From: Guido van Rossum <guido@python.org>
Date: Mon, 8 Sep 1997 02:20:57 +0000
Subject: [PATCH] Some nits, added a whole section on what you could do with
 metaclasses, and acks.

---
 Demo/metaclasses/index.html | 93 +++++++++++++++++++++++++++++++++++--
 1 file changed, 90 insertions(+), 3 deletions(-)

diff --git a/Demo/metaclasses/index.html b/Demo/metaclasses/index.html
index 269dc6938a5..ee91f8f1a22 100644
--- a/Demo/metaclasses/index.html
+++ b/Demo/metaclasses/index.html
@@ -23,7 +23,7 @@ <H4>XXX This is very much a work in progress.</H4>
 Fulton has used it in his <A
 HREF="http://www.digicool.com/papers/ExtensionClass.html">Extension
 Classes</A> package.  (It has also been referred to as the ``Don
-Beaudry <i>hack</i>, but that's a misnomer.  There's nothing hackish
+Beaudry <i>hack</i>,'' but that's a misnomer.  There's nothing hackish
 about it -- in fact, it is rather elegant and deep, even though
 there's something dark to it.)
 
@@ -182,6 +182,11 @@ <H2>Writing Metaclasses in Python</H2>
 in the future they will really be the same thing (at which point you
 would be able to derive subclasses from built-in types).
 
+<P>At this point it may be worth mentioning that C.__class__ is the
+same object as B.__class__, i.e., C's metaclass is the same as B's
+metaclass.  In other words, subclassing an existing class creates a
+new (meta)inststance of the base class's metaclass.
+
 <P>Going back to the example, the class B.__class__ is instantiated,
 passing its constructor the same three arguments that are passed to
 the default class constructor or to an extension's metaprogramming
@@ -229,7 +234,7 @@ <H2>Writing Metaclasses in Python</H2>
 
 <PRE>
 x = MySpecialClass()
-y = Myspecialclass()
+y = MySpecialClass()
 print x.__class__, y.__class__
 </PRE>
 
@@ -468,7 +473,7 @@ <H1>Real-life Examples</H1>
 <P>
 
 <DT><A HREF="Eiffel.py">Eiffel.py</A>
-ppp
+
 <DD>Uses the above generic metaclass to implement Eiffel style
 pre-conditions and post-conditions.
 
@@ -481,6 +486,12 @@ <H1>Real-life Examples</H1>
 
 <P>
 
+<DT><A HREF="Simple.py">Simple.py</A>
+
+<DD>The example module used above.
+
+<P>
+
 </DL>
 
 <P>A pattern seems to be emerging: almost all these uses of
@@ -493,6 +504,82 @@ <H1>Real-life Examples</H1>
 as well.  This needs more research.  Perhaps a metaclass could be
 provided that allows stackable wrappers...
 
+<HR>
+
+<H2>Things You Could Do With Metaclasses</H2>
+
+<P>There are lots of things you could do with metaclasses.  Most of
+these can also be done with creative use of __getattr__, but
+metaclasses make it easier to modify the attribute lookup behavior of
+classes.  Here's a partial list.
+
+<P>
+
+<UL>
+
+<LI>Enforce different inheritance semantics, e.g. automatically call
+base class methods when a derived class overrides<P>
+
+<LI>Implement class methods (e.g. if the first argument is not named
+'self')<P>
+
+<LI>Implement that each instance is initialized with <b>copies</b> of
+all class variables<P>
+
+<LI>Implement a different way to store instance variables (e.g. in a
+list kept outside the the instance but indexed by the instance's id())<P>
+
+<LI>Automatically wrap or trap all or certain methods
+
+<UL>
+
+<LI>for tracing
+
+<LI>for precondition and postcondition checking
+
+<LI>for synchronized methods
+
+<LI>for automatic value caching
+
+</UL>
+<P>
+
+<LI>When an attribute is a parameterless function, call it on
+reference (to mimic it being an instance variable); same on assignment<P>
+
+<LI>Instrumentation: see how many times various attributes are used<P>
+
+<LI>Different semantics for __setattr__ and __getattr__ (e.g.  disable
+them when they are being used recursively)<P>
+
+<LI>Abuse class syntax for other things<P>
+
+<LI>Experiment with automatic type checking<P>
+
+<LI>Delegation (or acquisition)<P>
+
+<LI>Dynamic inheritance patterns<P>
+
+<LI>Automatic caching of methods<P>
+
+</UL>
+
+<P>
+
+<HR>
+
+<H4>Credits</H4>
+
+<P>Many thanks to David Ascher and Donald Beaudry for their comments
+on earlier draft of this paper.  Also thanks to Matt Conway and Tommy
+Burnette for putting a seed for the idea of metaclasses in my
+mind, nearly three years ago, even though at the time my response was
+``you can do that with __getattr__ hooks...'' :-)
+
+<P>
+
+<HR>
+
 </BODY>
 
 </HTML>