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| 3 | <HEAD>
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| 4 | <TITLE>Metaclasses in Python 1.5</TITLE>
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| 5 | </HEAD>
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| 6 |
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| 7 | <BODY BGCOLOR="FFFFFF">
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| 8 |
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| 9 | <H1>Metaclasses in Python 1.5</H1>
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| 10 | <H2>(A.k.a. The Killer Joke :-)</H2>
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| 11 |
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| 12 | <HR>
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| 13 |
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| 14 | (<i>Postscript:</i> reading this essay is probably not the best way to
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| 15 | understand the metaclass hook described here. See a <A
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| 16 | HREF="meta-vladimir.txt">message posted by Vladimir Marangozov</A>
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| 17 | which may give a gentler introduction to the matter. You may also
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| 18 | want to search Deja News for messages with "metaclass" in the subject
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| 19 | posted to comp.lang.python in July and August 1998.)
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| 20 |
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| 21 | <HR>
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| 22 |
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| 23 | <P>In previous Python releases (and still in 1.5), there is something
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| 24 | called the ``Don Beaudry hook'', after its inventor and champion.
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| 25 | This allows C extensions to provide alternate class behavior, thereby
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| 26 | allowing the Python class syntax to be used to define other class-like
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| 27 | entities. Don Beaudry has used this in his infamous <A
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| 28 | HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> package; Jim
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| 29 | Fulton has used it in his <A
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| 30 | HREF="http://www.digicool.com/releases/ExtensionClass/">Extension
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| 31 | Classes</A> package. (It has also been referred to as the ``Don
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| 32 | Beaudry <i>hack</i>,'' but that's a misnomer. There's nothing hackish
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| 33 | about it -- in fact, it is rather elegant and deep, even though
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| 34 | there's something dark to it.)
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| 35 |
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| 36 | <P>(On first reading, you may want to skip directly to the examples in
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| 37 | the section "Writing Metaclasses in Python" below, unless you want
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| 38 | your head to explode.)
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| 39 |
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| 40 | <P>
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| 41 |
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| 42 | <HR>
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| 43 |
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| 44 | <P>Documentation of the Don Beaudry hook has purposefully been kept
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| 45 | minimal, since it is a feature of incredible power, and is easily
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| 46 | abused. Basically, it checks whether the <b>type of the base
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| 47 | class</b> is callable, and if so, it is called to create the new
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| 48 | class.
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| 49 |
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| 50 | <P>Note the two indirection levels. Take a simple example:
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| 51 |
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| 52 | <PRE>
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| 53 | class B:
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| 54 | pass
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| 55 |
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| 56 | class C(B):
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| 57 | pass
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| 58 | </PRE>
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| 59 |
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| 60 | Take a look at the second class definition, and try to fathom ``the
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| 61 | type of the base class is callable.''
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| 62 |
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| 63 | <P>(Types are not classes, by the way. See questions 4.2, 4.19 and in
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| 64 | particular 6.22 in the <A
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| 65 | HREF="http://www.python.org/cgi-bin/faqw.py" >Python FAQ</A>
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| 66 | for more on this topic.)
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| 67 |
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| 68 | <P>
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