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As Paul Prescod pointed out, metaprogramming is really something 

different (programs that write programs).  We are dealing with
metaclasses here.  So change the words slightly.
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Guido van Rossum 1997-09-11 22:54:49 +00:00
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@ -1,18 +1,16 @@
<HTML>
<HEAD>
<TITLE>Metaprogramming in Python 1.5</TITLE>
<TITLE>Metaclasses in Python 1.5</TITLE>
</HEAD>
<BODY BGCOLOR="FFFFFF">
<H1>Metaprogramming in Python 1.5 (DRAFT)</H1>
<H1>Metaclasses in Python 1.5</H1>
<H2>(A.k.a. The Killer Joke :-)</H2>
<H4>XXX This is very much a work in progress.</H4>
<P>While Python 1.5 is only out as a <A
HREF="http://grail.cnri.reston.va.us/python/1.5a3/">restricted alpha
release</A>, its metaprogramming feature is worth mentioning.
<P><b>Note: this document describes a feature only released in <A
HREF="../../1.5a3/">Python 1.5a3</A>.</b>
<P>In previous Python releases (and still in 1.5), there is something
called the ``Don Beaudry hook'', after its inventor and champion.
@ -144,7 +142,7 @@ class C:
the class namespace's contents would be {'a': 1, 'f': &lt;function f
...&gt;}.
<P>But enough already about Python metaprogramming in C; read the
<P>But enough already about writing Python metaclasses in C; read the
documentation of <A
HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> or <A
HREF="http://www.digicool.com/papers/ExtensionClass.html" >Extension
@ -153,7 +151,7 @@ Classes</A> for more information.
<H2>Writing Metaclasses in Python</H2>
<P>In Python 1.5, the requirement to write a C extension in order to
engage in metaprogramming has been dropped (though you can still do
write metaclasses has been dropped (though you can still do
it, of course). In addition to the check ``is the type of the base
class callable,'' there's a check ``does the base class have a
__class__ attribute.'' If so, it is assumed that the __class__
@ -189,8 +187,8 @@ 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
code: <i>name</i>, <i>bases</i>, and <i>namespace</i>.
the default class constructor or to an extension's metaclass:
<i>name</i>, <i>bases</i>, and <i>namespace</i>.
<P>It is easy to be confused by what exactly happens when using a
metaclass, because we lose the absolute distinction between classes
@ -244,7 +242,7 @@ How did this happen? MySpecialClass is an instance of MetaClass1
__call__ method is invoked, and presumably the __call__ method defined
by MetaClass1 returns an instance of HelperClass1.
<P>Now let's see how we could use metaprogramming -- what can we do
<P>Now let's see how we could use metaclasses -- what can we do
with metaclasses that we can't easily do without them? Here's one
idea: a metaclass could automatically insert trace calls for all
method calls. Let's first develop a simplified example, without
@ -424,7 +422,7 @@ doesn't support all these features yet.
<H1>Real-life Examples</H1>
<P>Have a look at some very preliminary examples that I coded up to
teach myself how to use metaprogramming:
teach myself how to write metaclasses:
<DL>