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| author | cinap_lenrek <cinap_lenrek@localhost> | 2011-05-03 11:25:13 +0000 |
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| committer | cinap_lenrek <cinap_lenrek@localhost> | 2011-05-03 11:25:13 +0000 |
| commit | 458120dd40db6b4df55a4e96b650e16798ef06a0 (patch) | |
| tree | 8f82685be24fef97e715c6f5ca4c68d34d5074ee /sys/src/cmd/python/Demo/metaclasses/index.html | |
| parent | 3a742c699f6806c1145aea5149bf15de15a0afd7 (diff) | |
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diff --git a/sys/src/cmd/python/Demo/metaclasses/index.html b/sys/src/cmd/python/Demo/metaclasses/index.html new file mode 100644 index 000000000..eee473a81 --- /dev/null +++ b/sys/src/cmd/python/Demo/metaclasses/index.html @@ -0,0 +1,605 @@ +<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> |