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diff --git a/Demo/metaclasses/index.html b/Demo/metaclasses/index.html deleted file mode 100644 index eee473a..0000000 --- a/Demo/metaclasses/index.html +++ /dev/null @@ -1,605 +0,0 @@ -<HTML> - -<HEAD> -<TITLE>Metaclasses in Python 1.5</TITLE> -</HEAD> - -<BODY BGCOLOR="FFFFFF"> - -<H1>Metaclasses in Python 1.5</H1> -<H2>(A.k.a. The Killer Joke :-)</H2> - -<HR> - -(<i>Postscript:</i> reading this essay is probably not the best way to -understand the metaclass hook described here. See a <A -HREF="meta-vladimir.txt">message posted by Vladimir Marangozov</A> -which may give a gentler introduction to the matter. You may also -want to search Deja News for messages with "metaclass" in the subject -posted to comp.lang.python in July and August 1998.) - -<HR> - -<P>In previous Python releases (and still in 1.5), there is something -called the ``Don Beaudry hook'', after its inventor and champion. -This allows C extensions to provide alternate class behavior, thereby -allowing the Python class syntax to be used to define other class-like -entities. Don Beaudry has used this in his infamous <A -HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> package; Jim -Fulton has used it in his <A -HREF="http://www.digicool.com/releases/ExtensionClass/">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 -about it -- in fact, it is rather elegant and deep, even though -there's something dark to it.) - -<P>(On first reading, you may want to skip directly to the examples in -the section "Writing Metaclasses in Python" below, unless you want -your head to explode.) - -<P> - -<HR> - -<P>Documentation of the Don Beaudry hook has purposefully been kept -minimal, since it is a feature of incredible power, and is easily -abused. Basically, it checks whether the <b>type of the base -class</b> is callable, and if so, it is called to create the new -class. - -<P>Note the two indirection levels. Take a simple example: - -<PRE> -class B: - pass - -class C(B): - pass -</PRE> - -Take a look at the second class definition, and try to fathom ``the -type of the base class is callable.'' - -<P>(Types are not classes, by the way. See questions 4.2, 4.19 and in -particular 6.22 in the <A -HREF="http://www.python.org/cgi-bin/faqw.py" >Python FAQ</A> -for more on this topic.) - -<P> - -<UL> - -<LI>The <b>base class</b> is B; this one's easy.<P> - -<LI>Since B is a class, its type is ``class''; so the <b>type of the -base class</b> is the type ``class''. This is also known as -types.ClassType, assuming the standard module <code>types</code> has -been imported.<P> - -<LI>Now is the type ``class'' <b>callable</b>? No, because types (in -core Python) are never callable. Classes are callable (calling a -class creates a new instance) but types aren't.<P> - -</UL> - -<P>So our conclusion is that in our example, the type of the base -class (of C) is not callable. So the Don Beaudry hook does not apply, -and the default class creation mechanism is used (which is also used -when there is no base class). In fact, the Don Beaudry hook never -applies when using only core Python, since the type of a core object -is never callable. - -<P>So what do Don and Jim do in order to use Don's hook? Write an -extension that defines at least two new Python object types. The -first would be the type for ``class-like'' objects usable as a base -class, to trigger Don's hook. This type must be made callable. -That's why we need a second type. Whether an object is callable -depends on its type. So whether a type object is callable depends on -<i>its</i> type, which is a <i>meta-type</i>. (In core Python there -is only one meta-type, the type ``type'' (types.TypeType), which is -the type of all type objects, even itself.) A new meta-type must -be defined that makes the type of the class-like objects callable. -(Normally, a third type would also be needed, the new ``instance'' -type, but this is not an absolute requirement -- the new class type -could return an object of some existing type when invoked to create an -instance.) - -<P>Still confused? Here's a simple device due to Don himself to -explain metaclasses. Take a simple class definition; assume B is a -special class that triggers Don's hook: - -<PRE> -class C(B): - a = 1 - b = 2 -</PRE> - -This can be though of as equivalent to: - -<PRE> -C = type(B)('C', (B,), {'a': 1, 'b': 2}) -</PRE> - -If that's too dense for you, here's the same thing written out using -temporary variables: - -<PRE> -creator = type(B) # The type of the base class -name = 'C' # The name of the new class -bases = (B,) # A tuple containing the base class(es) -namespace = {'a': 1, 'b': 2} # The namespace of the class statement -C = creator(name, bases, namespace) -</PRE> - -This is analogous to what happens without the Don Beaudry hook, except -that in that case the creator function is set to the default class -creator. - -<P>In either case, the creator is called with three arguments. The -first one, <i>name</i>, is the name of the new class (as given at the -top of the class statement). The <i>bases</i> argument is a tuple of -base classes (a singleton tuple if there's only one base class, like -the example). Finally, <i>namespace</i> is a dictionary containing -the local variables collected during execution of the class statement. - -<P>Note that the contents of the namespace dictionary is simply -whatever names were defined in the class statement. A little-known -fact is that when Python executes a class statement, it enters a new -local namespace, and all assignments and function definitions take -place in this namespace. Thus, after executing the following class -statement: - -<PRE> -class C: - a = 1 - def f(s): pass -</PRE> - -the class namespace's contents would be {'a': 1, 'f': <function f -...>}. - -<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 -Classes</A> for more information. - -<P> - -<HR> - -<H2>Writing Metaclasses in Python</H2> - -<P>In Python 1.5, the requirement to write a C extension in order to -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__ -attribute refers to a class. - -<P>Let's repeat our simple example from above: - -<PRE> -class C(B): - a = 1 - b = 2 -</PRE> - -Assuming B has a __class__ attribute, this translates into: - -<PRE> -C = B.__class__('C', (B,), {'a': 1, 'b': 2}) -</PRE> - -This is exactly the same as before except that instead of type(B), -B.__class__ is invoked. If you have read <A HREF= -"http://www.python.org/cgi-bin/faqw.py?req=show&file=faq06.022.htp" ->FAQ question 6.22</A> you will understand that while there is a big -technical difference between type(B) and B.__class__, they play the -same role at different abstraction levels. And perhaps at some point -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 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 -and instances: a class is an instance of a metaclass (a -``metainstance''), but technically (i.e. in the eyes of the python -runtime system), the metaclass is just a class, and the metainstance -is just an instance. At the end of the class statement, the metaclass -whose metainstance is used as a base class is instantiated, yielding a -second metainstance (of the same metaclass). This metainstance is -then used as a (normal, non-meta) class; instantiation of the class -means calling the metainstance, and this will return a real instance. -And what class is that an instance of? Conceptually, it is of course -an instance of our metainstance; but in most cases the Python runtime -system will see it as an instance of a a helper class used by the -metaclass to implement its (non-meta) instances... - -<P>Hopefully an example will make things clearer. Let's presume we -have a metaclass MetaClass1. It's helper class (for non-meta -instances) is callled HelperClass1. We now (manually) instantiate -MetaClass1 once to get an empty special base class: - -<PRE> -BaseClass1 = MetaClass1("BaseClass1", (), {}) -</PRE> - -We can now use BaseClass1 as a base class in a class statement: - -<PRE> -class MySpecialClass(BaseClass1): - i = 1 - def f(s): pass -</PRE> - -At this point, MySpecialClass is defined; it is a metainstance of -MetaClass1 just like BaseClass1, and in fact the expression -``BaseClass1.__class__ == MySpecialClass.__class__ == MetaClass1'' -yields true. - -<P>We are now ready to create instances of MySpecialClass. Let's -assume that no constructor arguments are required: - -<PRE> -x = MySpecialClass() -y = MySpecialClass() -print x.__class__, y.__class__ -</PRE> - -The print statement shows that x and y are instances of HelperClass1. -How did this happen? MySpecialClass is an instance of MetaClass1 -(``meta'' is irrelevant here); when an instance is called, its -__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 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 -support for inheritance or other ``advanced'' Python features (we'll -add those later). - -<PRE> -import types - -class Tracing: - def __init__(self, name, bases, namespace): - """Create a new class.""" - self.__name__ = name - self.__bases__ = bases - self.__namespace__ = namespace - def __call__(self): - """Create a new instance.""" - return Instance(self) - -class Instance: - def __init__(self, klass): - self.__klass__ = klass - def __getattr__(self, name): - try: - value = self.__klass__.__namespace__[name] - except KeyError: - raise AttributeError, name - if type(value) is not types.FunctionType: - return value - return BoundMethod(value, self) - -class BoundMethod: - def __init__(self, function, instance): - self.function = function - self.instance = instance - def __call__(self, *args): - print "calling", self.function, "for", self.instance, "with", args - return apply(self.function, (self.instance,) + args) - -Trace = Tracing('Trace', (), {}) - -class MyTracedClass(Trace): - def method1(self, a): - self.a = a - def method2(self): - return self.a - -aninstance = MyTracedClass() - -aninstance.method1(10) - -print "the answer is %d" % aninstance.method2() -</PRE> - -Confused already? The intention is to read this from top down. The -Tracing class is the metaclass we're defining. Its structure is -really simple. - -<P> - -<UL> - -<LI>The __init__ method is invoked when a new Tracing instance is -created, e.g. the definition of class MyTracedClass later in the -example. It simply saves the class name, base classes and namespace -as instance variables.<P> - -<LI>The __call__ method is invoked when a Tracing instance is called, -e.g. the creation of aninstance later in the example. It returns an -instance of the class Instance, which is defined next.<P> - -</UL> - -<P>The class Instance is the class used for all instances of classes -built using the Tracing metaclass, e.g. aninstance. It has two -methods: - -<P> - -<UL> - -<LI>The __init__ method is invoked from the Tracing.__call__ method -above to initialize a new instance. It saves the class reference as -an instance variable. It uses a funny name because the user's -instance variables (e.g. self.a later in the example) live in the same -namespace.<P> - -<LI>The __getattr__ method is invoked whenever the user code -references an attribute of the instance that is not an instance -variable (nor a class variable; but except for __init__ and -__getattr__ there are no class variables). It will be called, for -example, when aninstance.method1 is referenced in the example, with -self set to aninstance and name set to the string "method1".<P> - -</UL> - -<P>The __getattr__ method looks the name up in the __namespace__ -dictionary. If it isn't found, it raises an AttributeError exception. -(In a more realistic example, it would first have to look through the -base classes as well.) If it is found, there are two possibilities: -it's either a function or it isn't. If it's not a function, it is -assumed to be a class variable, and its value is returned. If it's a -function, we have to ``wrap'' it in instance of yet another helper -class, BoundMethod. - -<P>The BoundMethod class is needed to implement a familiar feature: -when a method is defined, it has an initial argument, self, which is -automatically bound to the relevant instance when it is called. For -example, aninstance.method1(10) is equivalent to method1(aninstance, -10). In the example if this call, first a temporary BoundMethod -instance is created with the following constructor call: temp = -BoundMethod(method1, aninstance); then this instance is called as -temp(10). After the call, the temporary instance is discarded. - -<P> - -<UL> - -<LI>The __init__ method is invoked for the constructor call -BoundMethod(method1, aninstance). It simply saves away its -arguments.<P> - -<LI>The __call__ method is invoked when the bound method instance is -called, as in temp(10). It needs to call method1(aninstance, 10). -However, even though self.function is now method1 and self.instance is -aninstance, it can't call self.function(self.instance, args) directly, -because it should work regardless of the number of arguments passed. -(For simplicity, support for keyword arguments has been omitted.)<P> - -</UL> - -<P>In order to be able to support arbitrary argument lists, the -__call__ method first constructs a new argument tuple. Conveniently, -because of the notation *args in __call__'s own argument list, the -arguments to __call__ (except for self) are placed in the tuple args. -To construct the desired argument list, we concatenate a singleton -tuple containing the instance with the args tuple: (self.instance,) + -args. (Note the trailing comma used to construct the singleton -tuple.) In our example, the resulting argument tuple is (aninstance, -10). - -<P>The intrinsic function apply() takes a function and an argument -tuple and calls the function for it. In our example, we are calling -apply(method1, (aninstance, 10)) which is equivalent to calling -method(aninstance, 10). - -<P>From here on, things should come together quite easily. The output -of the example code is something like this: - -<PRE> -calling <function method1 at ae8d8> for <Instance instance at 95ab0> with (10,) -calling <function method2 at ae900> for <Instance instance at 95ab0> with () -the answer is 10 -</PRE> - -<P>That was about the shortest meaningful example that I could come up -with. A real tracing metaclass (for example, <A -HREF="#Trace">Trace.py</A> discussed below) needs to be more -complicated in two dimensions. - -<P>First, it needs to support more advanced Python features such as -class variables, inheritance, __init__ methods, and keyword arguments. - -<P>Second, it needs to provide a more flexible way to handle the -actual tracing information; perhaps it should be possible to write -your own tracing function that gets called, perhaps it should be -possible to enable and disable tracing on a per-class or per-instance -basis, and perhaps a filter so that only interesting calls are traced; -it should also be able to trace the return value of the call (or the -exception it raised if an error occurs). Even the Trace.py example -doesn't support all these features yet. - -<P> - -<HR> - -<H1>Real-life Examples</H1> - -<P>Have a look at some very preliminary examples that I coded up to -teach myself how to write metaclasses: - -<DL> - -<DT><A HREF="Enum.py">Enum.py</A> - -<DD>This (ab)uses the class syntax as an elegant way to define -enumerated types. The resulting classes are never instantiated -- -rather, their class attributes are the enumerated values. For -example: - -<PRE> -class Color(Enum): - red = 1 - green = 2 - blue = 3 -print Color.red -</PRE> - -will print the string ``Color.red'', while ``Color.red==1'' is true, -and ``Color.red + 1'' raise a TypeError exception. - -<P> - -<DT><A NAME=Trace></A><A HREF="Trace.py">Trace.py</A> - -<DD>The resulting classes work much like standard -classes, but by setting a special class or instance attribute -__trace_output__ to point to a file, all calls to the class's methods -are traced. It was a bit of a struggle to get this right. This -should probably redone using the generic metaclass below. - -<P> - -<DT><A HREF="Meta.py">Meta.py</A> - -<DD>A generic metaclass. This is an attempt at finding out how much -standard class behavior can be mimicked by a metaclass. The -preliminary answer appears to be that everything's fine as long as the -class (or its clients) don't look at the instance's __class__ -attribute, nor at the class's __dict__ attribute. The use of -__getattr__ internally makes the classic implementation of __getattr__ -hooks tough; we provide a similar hook _getattr_ instead. -(__setattr__ and __delattr__ are not affected.) -(XXX Hm. Could detect presence of __getattr__ and rename it.) - -<P> - -<DT><A HREF="Eiffel.py">Eiffel.py</A> - -<DD>Uses the above generic metaclass to implement Eiffel style -pre-conditions and post-conditions. - -<P> - -<DT><A HREF="Synch.py">Synch.py</A> - -<DD>Uses the above generic metaclass to implement synchronized -methods. - -<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 -metaclasses (except for Enum, which is probably more cute than useful) -mostly work by placing wrappers around method calls. An obvious -problem with that is that it's not easy to combine the features of -different metaclasses, while this would actually be quite useful: for -example, I wouldn't mind getting a trace from the test run of the -Synch module, and it would be interesting to add preconditions to it -as well. This needs more research. Perhaps a metaclass could be -provided that allows stackable wrappers... - -<P> - -<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 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> |