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Kaydet (Commit) 7ade6da8 authored tarafından Guido van Rossum's avatar Guido van Rossum
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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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Demo/metaclasses/index.html
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<HTML> <HTML>
<HEAD> <HEAD>
<TITLE>Metaprogramming in Python 1.5</TITLE> <TITLE>Metaclasses in Python 1.5</TITLE>
</HEAD> </HEAD>
<BODY BGCOLOR="FFFFFF"> <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><b>Note: this document describes a feature only released in <A
HREF="../../1.5a3/">Python 1.5a3</A>.</b>
<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>In previous Python releases (and still in 1.5), there is something <P>In previous Python releases (and still in 1.5), there is something
called the ``Don Beaudry hook'', after its inventor and champion. called the ``Don Beaudry hook'', after its inventor and champion.
...@@ -144,7 +142,7 @@ class C: ...@@ -144,7 +142,7 @@ class C:
the class namespace's contents would be {'a': 1, 'f': &lt;function f the class namespace's contents would be {'a': 1, 'f': &lt;function f
...&gt;}. ...&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 documentation of <A
HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> or <A HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> or <A
HREF="http://www.digicool.com/papers/ExtensionClass.html" >Extension HREF="http://www.digicool.com/papers/ExtensionClass.html" >Extension
...@@ -153,7 +151,7 @@ Classes</A> for more information. ...@@ -153,7 +151,7 @@ Classes</A> for more information.
<H2>Writing Metaclasses in Python</H2> <H2>Writing Metaclasses in Python</H2>
<P>In Python 1.5, the requirement to write a C extension in order to <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 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 callable,'' there's a check ``does the base class have a
__class__ attribute.'' If so, it is assumed that the __class__ __class__ attribute.'' If so, it is assumed that the __class__
...@@ -189,8 +187,8 @@ new (meta)inststance of the base class's metaclass. ...@@ -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, <P>Going back to the example, the class B.__class__ is instantiated,
passing its constructor the same three arguments that are passed to passing its constructor the same three arguments that are passed to
the default class constructor or to an extension's metaprogramming the default class constructor or to an extension's metaclass:
code: <i>name</i>, <i>bases</i>, and <i>namespace</i>. <i>name</i>, <i>bases</i>, and <i>namespace</i>.
<P>It is easy to be confused by what exactly happens when using a <P>It is easy to be confused by what exactly happens when using a
metaclass, because we lose the absolute distinction between classes metaclass, because we lose the absolute distinction between classes
...@@ -244,7 +242,7 @@ How did this happen? MySpecialClass is an instance of MetaClass1 ...@@ -244,7 +242,7 @@ How did this happen? MySpecialClass is an instance of MetaClass1
__call__ method is invoked, and presumably the __call__ method defined __call__ method is invoked, and presumably the __call__ method defined
by MetaClass1 returns an instance of HelperClass1. 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 with metaclasses that we can't easily do without them? Here's one
idea: a metaclass could automatically insert trace calls for all idea: a metaclass could automatically insert trace calls for all
method calls. Let's first develop a simplified example, without method calls. Let's first develop a simplified example, without
...@@ -424,7 +422,7 @@ doesn't support all these features yet. ...@@ -424,7 +422,7 @@ doesn't support all these features yet.
<H1>Real-life Examples</H1> <H1>Real-life Examples</H1>
<P>Have a look at some very preliminary examples that I coded up to <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> <DL>
......
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