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Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	1	<HTML>
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Guido van Rossum	7ade6da	1997-09-11 22:54:49 +0000	[diff] [blame]	4	<TITLE>Metaclasses in Python 1.5</TITLE>
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	5	</HEAD>
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Guido van Rossum	7ade6da	1997-09-11 22:54:49 +0000	[diff] [blame]	9	<H1>Metaclasses in Python 1.5</H1>
				10	<H2>(A.k.a. The Killer Joke :-)</H2>
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	11
Guido van Rossum	d895749	1998-08-10 01:54:40 +0000	[diff] [blame]	12	<HR>
				13
				14	(<i>Postscript:</i> reading this essay is probably not the best way to
				15	understand the metaclass hook described here. See a <A
				16	HREF="meta-vladimir.txt">message posted by Vladimir Marangozov</A>
				17	which may give a gentler introduction to the matter. You may also
				18	want to search Deja News for messages with "metaclass" in the subject
				19	posted to comp.lang.python in July and August 1998.)
				20
				21	<HR>
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	22
				23	<P>In previous Python releases (and still in 1.5), there is something
				24	called the ``Don Beaudry hook'', after its inventor and champion.
				25	This allows C extensions to provide alternate class behavior, thereby
				26	allowing the Python class syntax to be used to define other class-like
				27	entities. Don Beaudry has used this in his infamous <A
				28	HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> package; Jim
				29	Fulton has used it in his <A
Guido van Rossum	e0a928d	2000-01-20 21:37:19 +0000	[diff] [blame]	30	HREF="http://www.digicool.com/releases/ExtensionClass/">Extension
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	31	Classes</A> package. (It has also been referred to as the ``Don
Guido van Rossum	7b877a9	1997-09-08 02:20:57 +0000	[diff] [blame]	32	Beaudry <i>hack</i>,'' but that's a misnomer. There's nothing hackish
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	33	about it -- in fact, it is rather elegant and deep, even though
				34	there's something dark to it.)
				35
Guido van Rossum	626a8d0	1997-09-11 23:01:04 +0000	[diff] [blame]	36	<P>(On first reading, you may want to skip directly to the examples in
				37	the section "Writing Metaclasses in Python" below, unless you want
Guido van Rossum	d895749	1998-08-10 01:54:40 +0000	[diff] [blame]	38	your head to explode.)
Guido van Rossum	626a8d0	1997-09-11 23:01:04 +0000	[diff] [blame]	39
				40	<P>
				41
				42	<HR>
				43
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	44	<P>Documentation of the Don Beaudry hook has purposefully been kept
				45	minimal, since it is a feature of incredible power, and is easily
				46	abused. Basically, it checks whether the <b>type of the base
				47	class</b> is callable, and if so, it is called to create the new
				48	class.
				49
				50	<P>Note the two indirection levels. Take a simple example:
				51
				52	<PRE>
				53	class B:
				54	pass
				55
				56	class C(B):
				57	pass
				58	</PRE>
				59
				60	Take a look at the second class definition, and try to fathom ``the
				61	type of the base class is callable.''
				62
				63	<P>(Types are not classes, by the way. See questions 4.2, 4.19 and in
				64	particular 6.22 in the <A
Guido van Rossum	e0a928d	2000-01-20 21:37:19 +0000	[diff] [blame]	65	HREF="http://www.python.org/cgi-bin/faqw.py" >Python FAQ</A>
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	66	for more on this topic.)
				67
				68	<P>
				69
				70	<UL>
				71
				72	<LI>The <b>base class</b> is B; this one's easy.<P>
				73
				74	<LI>Since B is a class, its type is ``class''; so the <b>type of the
				75	base class</b> is the type ``class''. This is also known as
				76	types.ClassType, assuming the standard module <code>types</code> has
				77	been imported.<P>
				78
				79	<LI>Now is the type ``class'' <b>callable</b>? No, because types (in
				80	core Python) are never callable. Classes are callable (calling a
				81	class creates a new instance) but types aren't.<P>
				82
				83	</UL>
				84
				85	<P>So our conclusion is that in our example, the type of the base
				86	class (of C) is not callable. So the Don Beaudry hook does not apply,
				87	and the default class creation mechanism is used (which is also used
				88	when there is no base class). In fact, the Don Beaudry hook never
				89	applies when using only core Python, since the type of a core object
				90	is never callable.
				91
				92	<P>So what do Don and Jim do in order to use Don's hook? Write an
				93	extension that defines at least two new Python object types. The
				94	first would be the type for ``class-like'' objects usable as a base
				95	class, to trigger Don's hook. This type must be made callable.
				96	That's why we need a second type. Whether an object is callable
				97	depends on its type. So whether a type object is callable depends on
				98	<i>its</i> type, which is a <i>meta-type</i>. (In core Python there
				99	is only one meta-type, the type ``type'' (types.TypeType), which is
				100	the type of all type objects, even itself.) A new meta-type must
				101	be defined that makes the type of the class-like objects callable.
				102	(Normally, a third type would also be needed, the new ``instance''
				103	type, but this is not an absolute requirement -- the new class type
				104	could return an object of some existing type when invoked to create an
				105	instance.)
				106
				107	<P>Still confused? Here's a simple device due to Don himself to
				108	explain metaclasses. Take a simple class definition; assume B is a
				109	special class that triggers Don's hook:
				110
				111	<PRE>
				112	class C(B):
				113	a = 1
				114	b = 2
				115	</PRE>
				116
				117	This can be though of as equivalent to:
				118
				119	<PRE>
				120	C = type(B)('C', (B,), {'a': 1, 'b': 2})
				121	</PRE>
				122
				123	If that's too dense for you, here's the same thing written out using
				124	temporary variables:
				125
				126	<PRE>
				127	creator = type(B) # The type of the base class
				128	name = 'C' # The name of the new class
				129	bases = (B,) # A tuple containing the base class(es)
				130	namespace = {'a': 1, 'b': 2} # The namespace of the class statement
				131	C = creator(name, bases, namespace)
				132	</PRE>
				133
				134	This is analogous to what happens without the Don Beaudry hook, except
				135	that in that case the creator function is set to the default class
				136	creator.
				137
				138	<P>In either case, the creator is called with three arguments. The
				139	first one, <i>name</i>, is the name of the new class (as given at the
				140	top of the class statement). The <i>bases</i> argument is a tuple of
				141	base classes (a singleton tuple if there's only one base class, like
				142	the example). Finally, <i>namespace</i> is a dictionary containing
				143	the local variables collected during execution of the class statement.
				144
				145	<P>Note that the contents of the namespace dictionary is simply
				146	whatever names were defined in the class statement. A little-known
				147	fact is that when Python executes a class statement, it enters a new
				148	local namespace, and all assignments and function definitions take
				149	place in this namespace. Thus, after executing the following class
				150	statement:
				151
				152	<PRE>
				153	class C:
				154	a = 1
				155	def f(s): pass
				156	</PRE>
				157
				158	the class namespace's contents would be {'a': 1, 'f': <function f
				159	...>}.
				160
Guido van Rossum	7ade6da	1997-09-11 22:54:49 +0000	[diff] [blame]	161	<P>But enough already about writing Python metaclasses in C; read the
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	162	documentation of <A
				163	HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> or <A
				164	HREF="http://www.digicool.com/papers/ExtensionClass.html" >Extension
				165	Classes</A> for more information.
				166
Guido van Rossum	626a8d0	1997-09-11 23:01:04 +0000	[diff] [blame]	167	<P>
				168
				169	<HR>
				170
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	171	<H2>Writing Metaclasses in Python</H2>
				172
				173	<P>In Python 1.5, the requirement to write a C extension in order to
Guido van Rossum	7ade6da	1997-09-11 22:54:49 +0000	[diff] [blame]	174	write metaclasses has been dropped (though you can still do
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	175	it, of course). In addition to the check ``is the type of the base
				176	class callable,'' there's a check ``does the base class have a
				177	__class__ attribute.'' If so, it is assumed that the __class__
				178	attribute refers to a class.
				179
				180	<P>Let's repeat our simple example from above:
				181
				182	<PRE>
				183	class C(B):
				184	a = 1
				185	b = 2
				186	</PRE>
				187
				188	Assuming B has a __class__ attribute, this translates into:
				189
				190	<PRE>
				191	C = B.__class__('C', (B,), {'a': 1, 'b': 2})
				192	</PRE>
				193
				194	This is exactly the same as before except that instead of type(B),
				195	B.__class__ is invoked. If you have read <A HREF=
Guido van Rossum	e0a928d	2000-01-20 21:37:19 +0000	[diff] [blame]	196	"http://www.python.org/cgi-bin/faqw.py?req=show&file=faq06.022.htp"
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	197	>FAQ question 6.22</A> you will understand that while there is a big
				198	technical difference between type(B) and B.__class__, they play the
				199	same role at different abstraction levels. And perhaps at some point
				200	in the future they will really be the same thing (at which point you
				201	would be able to derive subclasses from built-in types).
				202
Guido van Rossum	7b877a9	1997-09-08 02:20:57 +0000	[diff] [blame]	203	<P>At this point it may be worth mentioning that C.__class__ is the
				204	same object as B.__class__, i.e., C's metaclass is the same as B's
				205	metaclass. In other words, subclassing an existing class creates a
				206	new (meta)inststance of the base class's metaclass.
				207
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	208	<P>Going back to the example, the class B.__class__ is instantiated,
				209	passing its constructor the same three arguments that are passed to
Guido van Rossum	7ade6da	1997-09-11 22:54:49 +0000	[diff] [blame]	210	the default class constructor or to an extension's metaclass:
				211	<i>name</i>, <i>bases</i>, and <i>namespace</i>.
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	212
				213	<P>It is easy to be confused by what exactly happens when using a
				214	metaclass, because we lose the absolute distinction between classes
				215	and instances: a class is an instance of a metaclass (a
				216	``metainstance''), but technically (i.e. in the eyes of the python
				217	runtime system), the metaclass is just a class, and the metainstance
				218	is just an instance. At the end of the class statement, the metaclass
				219	whose metainstance is used as a base class is instantiated, yielding a
				220	second metainstance (of the same metaclass). This metainstance is
				221	then used as a (normal, non-meta) class; instantiation of the class
				222	means calling the metainstance, and this will return a real instance.
				223	And what class is that an instance of? Conceptually, it is of course
				224	an instance of our metainstance; but in most cases the Python runtime
				225	system will see it as an instance of a a helper class used by the
				226	metaclass to implement its (non-meta) instances...
				227
				228	<P>Hopefully an example will make things clearer. Let's presume we
				229	have a metaclass MetaClass1. It's helper class (for non-meta
				230	instances) is callled HelperClass1. We now (manually) instantiate
				231	MetaClass1 once to get an empty special base class:
				232
				233	<PRE>
				234	BaseClass1 = MetaClass1("BaseClass1", (), {})
				235	</PRE>
				236
				237	We can now use BaseClass1 as a base class in a class statement:
				238
				239	<PRE>
				240	class MySpecialClass(BaseClass1):
				241	i = 1
				242	def f(s): pass
				243	</PRE>
				244
				245	At this point, MySpecialClass is defined; it is a metainstance of
				246	MetaClass1 just like BaseClass1, and in fact the expression
				247	``BaseClass1.__class__ == MySpecialClass.__class__ == MetaClass1''
				248	yields true.
				249
				250	<P>We are now ready to create instances of MySpecialClass. Let's
				251	assume that no constructor arguments are required:
				252
				253	<PRE>
				254	x = MySpecialClass()
Guido van Rossum	7b877a9	1997-09-08 02:20:57 +0000	[diff] [blame]	255	y = MySpecialClass()
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	256	print x.__class__, y.__class__
				257	</PRE>
				258
				259	The print statement shows that x and y are instances of HelperClass1.
				260	How did this happen? MySpecialClass is an instance of MetaClass1
				261	(``meta'' is irrelevant here); when an instance is called, its
				262	__call__ method is invoked, and presumably the __call__ method defined
				263	by MetaClass1 returns an instance of HelperClass1.
				264
Guido van Rossum	7ade6da	1997-09-11 22:54:49 +0000	[diff] [blame]	265	<P>Now let's see how we could use metaclasses -- what can we do
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	266	with metaclasses that we can't easily do without them? Here's one
				267	idea: a metaclass could automatically insert trace calls for all
				268	method calls. Let's first develop a simplified example, without
				269	support for inheritance or other ``advanced'' Python features (we'll
				270	add those later).
				271
				272	<PRE>
				273	import types
				274
				275	class Tracing:
				276	def __init__(self, name, bases, namespace):
				277	"""Create a new class."""
				278	self.__name__ = name
				279	self.__bases__ = bases
				280	self.__namespace__ = namespace
				281	def __call__(self):
				282	"""Create a new instance."""
				283	return Instance(self)
				284
				285	class Instance:
				286	def __init__(self, klass):
				287	self.__klass__ = klass
				288	def __getattr__(self, name):
				289	try:
				290	value = self.__klass__.__namespace__[name]
				291	except KeyError:
				292	raise AttributeError, name
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	293	if type(value) is not types.FunctionType:
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	294	return value
				295	return BoundMethod(value, self)
				296
				297	class BoundMethod:
				298	def __init__(self, function, instance):
				299	self.function = function
				300	self.instance = instance
				301	def __call__(self, *args):
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	302	print "calling", self.function, "for", self.instance, "with", args
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	303	return apply(self.function, (self.instance,) + args)
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	304
				305	Trace = Tracing('Trace', (), {})
				306
				307	class MyTracedClass(Trace):
				308	def method1(self, a):
				309	self.a = a
				310	def method2(self):
				311	return self.a
				312
				313	aninstance = MyTracedClass()
				314
				315	aninstance.method1(10)
				316
				317	print "the answer is %d" % aninstance.method2()
				318	</PRE>
				319
				320	Confused already? The intention is to read this from top down. The
				321	Tracing class is the metaclass we're defining. Its structure is
				322	really simple.
				323
				324	<P>
				325
				326	<UL>
				327
				328	<LI>The __init__ method is invoked when a new Tracing instance is
				329	created, e.g. the definition of class MyTracedClass later in the
				330	example. It simply saves the class name, base classes and namespace
				331	as instance variables.<P>
				332
				333	<LI>The __call__ method is invoked when a Tracing instance is called,
				334	e.g. the creation of aninstance later in the example. It returns an
				335	instance of the class Instance, which is defined next.<P>
				336
				337	</UL>
				338
				339	<P>The class Instance is the class used for all instances of classes
				340	built using the Tracing metaclass, e.g. aninstance. It has two
				341	methods:
				342
				343	<P>
				344
				345	<UL>
				346
				347	<LI>The __init__ method is invoked from the Tracing.__call__ method
				348	above to initialize a new instance. It saves the class reference as
				349	an instance variable. It uses a funny name because the user's
				350	instance variables (e.g. self.a later in the example) live in the same
				351	namespace.<P>
				352
				353	<LI>The __getattr__ method is invoked whenever the user code
				354	references an attribute of the instance that is not an instance
				355	variable (nor a class variable; but except for __init__ and
				356	__getattr__ there are no class variables). It will be called, for
				357	example, when aninstance.method1 is referenced in the example, with
				358	self set to aninstance and name set to the string "method1".<P>
				359
				360	</UL>
				361
				362	<P>The __getattr__ method looks the name up in the __namespace__
				363	dictionary. If it isn't found, it raises an AttributeError exception.
				364	(In a more realistic example, it would first have to look through the
				365	base classes as well.) If it is found, there are two possibilities:
				366	it's either a function or it isn't. If it's not a function, it is
				367	assumed to be a class variable, and its value is returned. If it's a
				368	function, we have to ``wrap'' it in instance of yet another helper
				369	class, BoundMethod.
				370
				371	<P>The BoundMethod class is needed to implement a familiar feature:
				372	when a method is defined, it has an initial argument, self, which is
				373	automatically bound to the relevant instance when it is called. For
				374	example, aninstance.method1(10) is equivalent to method1(aninstance,
				375	10). In the example if this call, first a temporary BoundMethod
				376	instance is created with the following constructor call: temp =
				377	BoundMethod(method1, aninstance); then this instance is called as
				378	temp(10). After the call, the temporary instance is discarded.
				379
				380	<P>
				381
				382	<UL>
				383
				384	<LI>The __init__ method is invoked for the constructor call
				385	BoundMethod(method1, aninstance). It simply saves away its
				386	arguments.<P>
				387
				388	<LI>The __call__ method is invoked when the bound method instance is
				389	called, as in temp(10). It needs to call method1(aninstance, 10).
				390	However, even though self.function is now method1 and self.instance is
				391	aninstance, it can't call self.function(self.instance, args) directly,
				392	because it should work regardless of the number of arguments passed.
				393	(For simplicity, support for keyword arguments has been omitted.)<P>
				394
				395	</UL>
				396
				397	<P>In order to be able to support arbitrary argument lists, the
				398	__call__ method first constructs a new argument tuple. Conveniently,
				399	because of the notation *args in __call__'s own argument list, the
				400	arguments to __call__ (except for self) are placed in the tuple args.
				401	To construct the desired argument list, we concatenate a singleton
				402	tuple containing the instance with the args tuple: (self.instance,) +
				403	args. (Note the trailing comma used to construct the singleton
				404	tuple.) In our example, the resulting argument tuple is (aninstance,
				405	10).
				406
				407	<P>The intrinsic function apply() takes a function and an argument
				408	tuple and calls the function for it. In our example, we are calling
				409	apply(method1, (aninstance, 10)) which is equivalent to calling
				410	method(aninstance, 10).
				411
				412	<P>From here on, things should come together quite easily. The output
				413	of the example code is something like this:
				414
				415	<PRE>
Guido van Rossum	c37eb56	1997-12-05 16:18:45 +0000	[diff] [blame]	416	calling <function method1 at ae8d8> for <Instance instance at 95ab0> with (10,)
				417	calling <function method2 at ae900> for <Instance instance at 95ab0> with ()
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	418	the answer is 10
				419	</PRE>
				420
				421	<P>That was about the shortest meaningful example that I could come up
				422	with. A real tracing metaclass (for example, <A
				423	HREF="#Trace">Trace.py</A> discussed below) needs to be more
				424	complicated in two dimensions.
				425
				426	<P>First, it needs to support more advanced Python features such as
				427	class variables, inheritance, __init__ methods, and keyword arguments.
				428
				429	<P>Second, it needs to provide a more flexible way to handle the
				430	actual tracing information; perhaps it should be possible to write
				431	your own tracing function that gets called, perhaps it should be
				432	possible to enable and disable tracing on a per-class or per-instance
				433	basis, and perhaps a filter so that only interesting calls are traced;
				434	it should also be able to trace the return value of the call (or the
				435	exception it raised if an error occurs). Even the Trace.py example
				436	doesn't support all these features yet.
				437
				438	<P>
				439
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	440	<HR>
				441
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	442	<H1>Real-life Examples</H1>
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	443
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	444	<P>Have a look at some very preliminary examples that I coded up to
Guido van Rossum	7ade6da	1997-09-11 22:54:49 +0000	[diff] [blame]	445	teach myself how to write metaclasses:
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	446
				447	<DL>
				448
				449	<DT><A HREF="Enum.py">Enum.py</A>
				450
				451	<DD>This (ab)uses the class syntax as an elegant way to define
				452	enumerated types. The resulting classes are never instantiated --
				453	rather, their class attributes are the enumerated values. For
				454	example:
				455
				456	<PRE>
				457	class Color(Enum):
				458	red = 1
				459	green = 2
				460	blue = 3
				461	print Color.red
				462	</PRE>
				463
				464	will print the string ``Color.red'', while ``Color.red==1'' is true,
				465	and ``Color.red + 1'' raise a TypeError exception.
				466
				467	<P>
				468
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	469	<DT><A NAME=Trace></A><A HREF="Trace.py">Trace.py</A>
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	470
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	471	<DD>The resulting classes work much like standard
				472	classes, but by setting a special class or instance attribute
				473	__trace_output__ to point to a file, all calls to the class's methods
				474	are traced. It was a bit of a struggle to get this right. This
				475	should probably redone using the generic metaclass below.
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	476
				477	<P>
				478
				479	<DT><A HREF="Meta.py">Meta.py</A>
				480
				481	<DD>A generic metaclass. This is an attempt at finding out how much
				482	standard class behavior can be mimicked by a metaclass. The
				483	preliminary answer appears to be that everything's fine as long as the
				484	class (or its clients) don't look at the instance's __class__
				485	attribute, nor at the class's __dict__ attribute. The use of
				486	__getattr__ internally makes the classic implementation of __getattr__
				487	hooks tough; we provide a similar hook _getattr_ instead.
				488	(__setattr__ and __delattr__ are not affected.)
				489	(XXX Hm. Could detect presence of __getattr__ and rename it.)
				490
				491	<P>
				492
				493	<DT><A HREF="Eiffel.py">Eiffel.py</A>
Guido van Rossum	7b877a9	1997-09-08 02:20:57 +0000	[diff] [blame]	494
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	495	<DD>Uses the above generic metaclass to implement Eiffel style
				496	pre-conditions and post-conditions.
				497
				498	<P>
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	499
				500	<DT><A HREF="Synch.py">Synch.py</A>
				501
				502	<DD>Uses the above generic metaclass to implement synchronized
				503	methods.
				504
				505	<P>
				506
Guido van Rossum	7b877a9	1997-09-08 02:20:57 +0000	[diff] [blame]	507	<DT><A HREF="Simple.py">Simple.py</A>
				508
				509	<DD>The example module used above.
				510
				511	<P>
				512
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	513	</DL>
				514
Guido van Rossum	0cdb887	1997-08-26 00:08:51 +0000	[diff] [blame]	515	<P>A pattern seems to be emerging: almost all these uses of
				516	metaclasses (except for Enum, which is probably more cute than useful)
				517	mostly work by placing wrappers around method calls. An obvious
				518	problem with that is that it's not easy to combine the features of
				519	different metaclasses, while this would actually be quite useful: for
				520	example, I wouldn't mind getting a trace from the test run of the
				521	Synch module, and it would be interesting to add preconditions to it
				522	as well. This needs more research. Perhaps a metaclass could be
				523	provided that allows stackable wrappers...
				524
Guido van Rossum	626a8d0	1997-09-11 23:01:04 +0000	[diff] [blame]	525	<P>
				526
Guido van Rossum	7b877a9	1997-09-08 02:20:57 +0000	[diff] [blame]	527	<HR>
				528
				529	<H2>Things You Could Do With Metaclasses</H2>
				530
				531	<P>There are lots of things you could do with metaclasses. Most of
				532	these can also be done with creative use of __getattr__, but
				533	metaclasses make it easier to modify the attribute lookup behavior of
				534	classes. Here's a partial list.
				535
				536	<P>
				537
				538	<UL>
				539
				540	<LI>Enforce different inheritance semantics, e.g. automatically call
				541	base class methods when a derived class overrides<P>
				542
				543	<LI>Implement class methods (e.g. if the first argument is not named
				544	'self')<P>
				545
				546	<LI>Implement that each instance is initialized with <b>copies</b> of
				547	all class variables<P>
				548
				549	<LI>Implement a different way to store instance variables (e.g. in a
				550	list kept outside the the instance but indexed by the instance's id())<P>
				551
				552	<LI>Automatically wrap or trap all or certain methods
				553
				554	<UL>
				555
				556	<LI>for tracing
				557
				558	<LI>for precondition and postcondition checking
				559
				560	<LI>for synchronized methods
				561
				562	<LI>for automatic value caching
				563
				564	</UL>
				565	<P>
				566
				567	<LI>When an attribute is a parameterless function, call it on
				568	reference (to mimic it being an instance variable); same on assignment<P>
				569
				570	<LI>Instrumentation: see how many times various attributes are used<P>
				571
				572	<LI>Different semantics for __setattr__ and __getattr__ (e.g. disable
				573	them when they are being used recursively)<P>
				574
				575	<LI>Abuse class syntax for other things<P>
				576
				577	<LI>Experiment with automatic type checking<P>
				578
				579	<LI>Delegation (or acquisition)<P>
				580
				581	<LI>Dynamic inheritance patterns<P>
				582
				583	<LI>Automatic caching of methods<P>
				584
				585	</UL>
				586
				587	<P>
				588
				589	<HR>
				590
				591	<H4>Credits</H4>
				592
				593	<P>Many thanks to David Ascher and Donald Beaudry for their comments
				594	on earlier draft of this paper. Also thanks to Matt Conway and Tommy
				595	Burnette for putting a seed for the idea of metaclasses in my
				596	mind, nearly three years ago, even though at the time my response was
				597	``you can do that with __getattr__ hooks...'' :-)
				598
				599	<P>
				600
				601	<HR>
				602
Guido van Rossum	1fb071c	1997-08-25 21:36:44 +0000	[diff] [blame]	603	</BODY>
				604
				605	</HTML>