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+<HEAD>\r
+<TITLE>Metaclasses in Python 1.5</TITLE>\r
+</HEAD>\r
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+<BODY BGCOLOR="FFFFFF">\r
+\r
+<H1>Metaclasses in Python 1.5</H1>\r
+<H2>(A.k.a. The Killer Joke :-)</H2>\r
+\r
+<HR>\r
+\r
+(<i>Postscript:</i> reading this essay is probably not the best way to\r
+understand the metaclass hook described here. See a <A\r
+HREF="meta-vladimir.txt">message posted by Vladimir Marangozov</A>\r
+which may give a gentler introduction to the matter. You may also\r
+want to search Deja News for messages with "metaclass" in the subject\r
+posted to comp.lang.python in July and August 1998.)\r
+\r
+<HR>\r
+\r
+<P>In previous Python releases (and still in 1.5), there is something\r
+called the ``Don Beaudry hook'', after its inventor and champion.\r
+This allows C extensions to provide alternate class behavior, thereby\r
+allowing the Python class syntax to be used to define other class-like\r
+entities. Don Beaudry has used this in his infamous <A\r
+HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> package; Jim\r
+Fulton has used it in his <A\r
+HREF="http://www.digicool.com/releases/ExtensionClass/">Extension\r
+Classes</A> package. (It has also been referred to as the ``Don\r
+Beaudry <i>hack</i>,'' but that's a misnomer. There's nothing hackish\r
+about it -- in fact, it is rather elegant and deep, even though\r
+there's something dark to it.)\r
+\r
+<P>(On first reading, you may want to skip directly to the examples in\r
+the section "Writing Metaclasses in Python" below, unless you want\r
+your head to explode.)\r
+\r
+<P>\r
+\r
+<HR>\r
+\r
+<P>Documentation of the Don Beaudry hook has purposefully been kept\r
+minimal, since it is a feature of incredible power, and is easily\r
+abused. Basically, it checks whether the <b>type of the base\r
+class</b> is callable, and if so, it is called to create the new\r
+class.\r
+\r
+<P>Note the two indirection levels. Take a simple example:\r
+\r
+<PRE>\r
+class B:\r
+ pass\r
+\r
+class C(B):\r
+ pass\r
+</PRE>\r
+\r
+Take a look at the second class definition, and try to fathom ``the\r
+type of the base class is callable.''\r
+\r
+<P>(Types are not classes, by the way. See questions 4.2, 4.19 and in\r
+particular 6.22 in the <A\r
+HREF="http://www.python.org/cgi-bin/faqw.py" >Python FAQ</A>\r
+for more on this topic.)\r
+\r
+<P>\r
+\r
+<UL>\r
+\r
+<LI>The <b>base class</b> is B; this one's easy.<P>\r
+\r
+<LI>Since B is a class, its type is ``class''; so the <b>type of the\r
+base class</b> is the type ``class''. This is also known as\r
+types.ClassType, assuming the standard module <code>types</code> has\r
+been imported.<P>\r
+\r
+<LI>Now is the type ``class'' <b>callable</b>? No, because types (in\r
+core Python) are never callable. Classes are callable (calling a\r
+class creates a new instance) but types aren't.<P>\r
+\r
+</UL>\r
+\r
+<P>So our conclusion is that in our example, the type of the base\r
+class (of C) is not callable. So the Don Beaudry hook does not apply,\r
+and the default class creation mechanism is used (which is also used\r
+when there is no base class). In fact, the Don Beaudry hook never\r
+applies when using only core Python, since the type of a core object\r
+is never callable.\r
+\r
+<P>So what do Don and Jim do in order to use Don's hook? Write an\r
+extension that defines at least two new Python object types. The\r
+first would be the type for ``class-like'' objects usable as a base\r
+class, to trigger Don's hook. This type must be made callable.\r
+That's why we need a second type. Whether an object is callable\r
+depends on its type. So whether a type object is callable depends on\r
+<i>its</i> type, which is a <i>meta-type</i>. (In core Python there\r
+is only one meta-type, the type ``type'' (types.TypeType), which is\r
+the type of all type objects, even itself.) A new meta-type must\r
+be defined that makes the type of the class-like objects callable.\r
+(Normally, a third type would also be needed, the new ``instance''\r
+type, but this is not an absolute requirement -- the new class type\r
+could return an object of some existing type when invoked to create an\r
+instance.)\r
+\r
+<P>Still confused? Here's a simple device due to Don himself to\r
+explain metaclasses. Take a simple class definition; assume B is a\r
+special class that triggers Don's hook:\r
+\r
+<PRE>\r
+class C(B):\r
+ a = 1\r
+ b = 2\r
+</PRE>\r
+\r
+This can be though of as equivalent to:\r
+\r
+<PRE>\r
+C = type(B)('C', (B,), {'a': 1, 'b': 2})\r
+</PRE>\r
+\r
+If that's too dense for you, here's the same thing written out using\r
+temporary variables:\r
+\r
+<PRE>\r
+creator = type(B) # The type of the base class\r
+name = 'C' # The name of the new class\r
+bases = (B,) # A tuple containing the base class(es)\r
+namespace = {'a': 1, 'b': 2} # The namespace of the class statement\r
+C = creator(name, bases, namespace)\r
+</PRE>\r
+\r
+This is analogous to what happens without the Don Beaudry hook, except\r
+that in that case the creator function is set to the default class\r
+creator.\r
+\r
+<P>In either case, the creator is called with three arguments. The\r
+first one, <i>name</i>, is the name of the new class (as given at the\r
+top of the class statement). The <i>bases</i> argument is a tuple of\r
+base classes (a singleton tuple if there's only one base class, like\r
+the example). Finally, <i>namespace</i> is a dictionary containing\r
+the local variables collected during execution of the class statement.\r
+\r
+<P>Note that the contents of the namespace dictionary is simply\r
+whatever names were defined in the class statement. A little-known\r
+fact is that when Python executes a class statement, it enters a new\r
+local namespace, and all assignments and function definitions take\r
+place in this namespace. Thus, after executing the following class\r
+statement:\r
+\r
+<PRE>\r
+class C:\r
+ a = 1\r
+ def f(s): pass\r
+</PRE>\r
+\r
+the class namespace's contents would be {'a': 1, 'f': <function f\r
+...>}.\r
+\r
+<P>But enough already about writing Python metaclasses in C; read the\r
+documentation of <A\r
+HREF="http://maigret.cog.brown.edu/pyutil/">MESS</A> or <A\r
+HREF="http://www.digicool.com/papers/ExtensionClass.html" >Extension\r
+Classes</A> for more information.\r
+\r
+<P>\r
+\r
+<HR>\r
+\r
+<H2>Writing Metaclasses in Python</H2>\r
+\r
+<P>In Python 1.5, the requirement to write a C extension in order to\r
+write metaclasses has been dropped (though you can still do\r
+it, of course). In addition to the check ``is the type of the base\r
+class callable,'' there's a check ``does the base class have a\r
+__class__ attribute.'' If so, it is assumed that the __class__\r
+attribute refers to a class.\r
+\r
+<P>Let's repeat our simple example from above:\r
+\r
+<PRE>\r
+class C(B):\r
+ a = 1\r
+ b = 2\r
+</PRE>\r
+\r
+Assuming B has a __class__ attribute, this translates into:\r
+\r
+<PRE>\r
+C = B.__class__('C', (B,), {'a': 1, 'b': 2})\r
+</PRE>\r
+\r
+This is exactly the same as before except that instead of type(B),\r
+B.__class__ is invoked. If you have read <A HREF=\r
+"http://www.python.org/cgi-bin/faqw.py?req=show&file=faq06.022.htp"\r
+>FAQ question 6.22</A> you will understand that while there is a big\r
+technical difference between type(B) and B.__class__, they play the\r
+same role at different abstraction levels. And perhaps at some point\r
+in the future they will really be the same thing (at which point you\r
+would be able to derive subclasses from built-in types).\r
+\r
+<P>At this point it may be worth mentioning that C.__class__ is the\r
+same object as B.__class__, i.e., C's metaclass is the same as B's\r
+metaclass. In other words, subclassing an existing class creates a\r
+new (meta)inststance of the base class's metaclass.\r
+\r
+<P>Going back to the example, the class B.__class__ is instantiated,\r
+passing its constructor the same three arguments that are passed to\r
+the default class constructor or to an extension's metaclass:\r
+<i>name</i>, <i>bases</i>, and <i>namespace</i>.\r
+\r
+<P>It is easy to be confused by what exactly happens when using a\r
+metaclass, because we lose the absolute distinction between classes\r
+and instances: a class is an instance of a metaclass (a\r
+``metainstance''), but technically (i.e. in the eyes of the python\r
+runtime system), the metaclass is just a class, and the metainstance\r
+is just an instance. At the end of the class statement, the metaclass\r
+whose metainstance is used as a base class is instantiated, yielding a\r
+second metainstance (of the same metaclass). This metainstance is\r
+then used as a (normal, non-meta) class; instantiation of the class\r
+means calling the metainstance, and this will return a real instance.\r
+And what class is that an instance of? Conceptually, it is of course\r
+an instance of our metainstance; but in most cases the Python runtime\r
+system will see it as an instance of a a helper class used by the\r
+metaclass to implement its (non-meta) instances...\r
+\r
+<P>Hopefully an example will make things clearer. Let's presume we\r
+have a metaclass MetaClass1. It's helper class (for non-meta\r
+instances) is callled HelperClass1. We now (manually) instantiate\r
+MetaClass1 once to get an empty special base class:\r
+\r
+<PRE>\r
+BaseClass1 = MetaClass1("BaseClass1", (), {})\r
+</PRE>\r
+\r
+We can now use BaseClass1 as a base class in a class statement:\r
+\r
+<PRE>\r
+class MySpecialClass(BaseClass1):\r
+ i = 1\r
+ def f(s): pass\r
+</PRE>\r
+\r
+At this point, MySpecialClass is defined; it is a metainstance of\r
+MetaClass1 just like BaseClass1, and in fact the expression\r
+``BaseClass1.__class__ == MySpecialClass.__class__ == MetaClass1''\r
+yields true.\r
+\r
+<P>We are now ready to create instances of MySpecialClass. Let's\r
+assume that no constructor arguments are required:\r
+\r
+<PRE>\r
+x = MySpecialClass()\r
+y = MySpecialClass()\r
+print x.__class__, y.__class__\r
+</PRE>\r
+\r
+The print statement shows that x and y are instances of HelperClass1.\r
+How did this happen? MySpecialClass is an instance of MetaClass1\r
+(``meta'' is irrelevant here); when an instance is called, its\r
+__call__ method is invoked, and presumably the __call__ method defined\r
+by MetaClass1 returns an instance of HelperClass1.\r
+\r
+<P>Now let's see how we could use metaclasses -- what can we do\r
+with metaclasses that we can't easily do without them? Here's one\r
+idea: a metaclass could automatically insert trace calls for all\r
+method calls. Let's first develop a simplified example, without\r
+support for inheritance or other ``advanced'' Python features (we'll\r
+add those later).\r
+\r
+<PRE>\r
+import types\r
+\r
+class Tracing:\r
+ def __init__(self, name, bases, namespace):\r
+ """Create a new class."""\r
+ self.__name__ = name\r
+ self.__bases__ = bases\r
+ self.__namespace__ = namespace\r
+ def __call__(self):\r
+ """Create a new instance."""\r
+ return Instance(self)\r
+\r
+class Instance:\r
+ def __init__(self, klass):\r
+ self.__klass__ = klass\r
+ def __getattr__(self, name):\r
+ try:\r
+ value = self.__klass__.__namespace__[name]\r
+ except KeyError:\r
+ raise AttributeError, name\r
+ if type(value) is not types.FunctionType:\r
+ return value\r
+ return BoundMethod(value, self)\r
+\r
+class BoundMethod:\r
+ def __init__(self, function, instance):\r
+ self.function = function\r
+ self.instance = instance\r
+ def __call__(self, *args):\r
+ print "calling", self.function, "for", self.instance, "with", args\r
+ return apply(self.function, (self.instance,) + args)\r
+\r
+Trace = Tracing('Trace', (), {})\r
+\r
+class MyTracedClass(Trace):\r
+ def method1(self, a):\r
+ self.a = a\r
+ def method2(self):\r
+ return self.a\r
+\r
+aninstance = MyTracedClass()\r
+\r
+aninstance.method1(10)\r
+\r
+print "the answer is %d" % aninstance.method2()\r
+</PRE>\r
+\r
+Confused already? The intention is to read this from top down. The\r
+Tracing class is the metaclass we're defining. Its structure is\r
+really simple.\r
+\r
+<P>\r
+\r
+<UL>\r
+\r
+<LI>The __init__ method is invoked when a new Tracing instance is\r
+created, e.g. the definition of class MyTracedClass later in the\r
+example. It simply saves the class name, base classes and namespace\r
+as instance variables.<P>\r
+\r
+<LI>The __call__ method is invoked when a Tracing instance is called,\r
+e.g. the creation of aninstance later in the example. It returns an\r
+instance of the class Instance, which is defined next.<P>\r
+\r
+</UL>\r
+\r
+<P>The class Instance is the class used for all instances of classes\r
+built using the Tracing metaclass, e.g. aninstance. It has two\r
+methods:\r
+\r
+<P>\r
+\r
+<UL>\r
+\r
+<LI>The __init__ method is invoked from the Tracing.__call__ method\r
+above to initialize a new instance. It saves the class reference as\r
+an instance variable. It uses a funny name because the user's\r
+instance variables (e.g. self.a later in the example) live in the same\r
+namespace.<P>\r
+\r
+<LI>The __getattr__ method is invoked whenever the user code\r
+references an attribute of the instance that is not an instance\r
+variable (nor a class variable; but except for __init__ and\r
+__getattr__ there are no class variables). It will be called, for\r
+example, when aninstance.method1 is referenced in the example, with\r
+self set to aninstance and name set to the string "method1".<P>\r
+\r
+</UL>\r
+\r
+<P>The __getattr__ method looks the name up in the __namespace__\r
+dictionary. If it isn't found, it raises an AttributeError exception.\r
+(In a more realistic example, it would first have to look through the\r
+base classes as well.) If it is found, there are two possibilities:\r
+it's either a function or it isn't. If it's not a function, it is\r
+assumed to be a class variable, and its value is returned. If it's a\r
+function, we have to ``wrap'' it in instance of yet another helper\r
+class, BoundMethod.\r
+\r
+<P>The BoundMethod class is needed to implement a familiar feature:\r
+when a method is defined, it has an initial argument, self, which is\r
+automatically bound to the relevant instance when it is called. For\r
+example, aninstance.method1(10) is equivalent to method1(aninstance,\r
+10). In the example if this call, first a temporary BoundMethod\r
+instance is created with the following constructor call: temp =\r
+BoundMethod(method1, aninstance); then this instance is called as\r
+temp(10). After the call, the temporary instance is discarded.\r
+\r
+<P>\r
+\r
+<UL>\r
+\r
+<LI>The __init__ method is invoked for the constructor call\r
+BoundMethod(method1, aninstance). It simply saves away its\r
+arguments.<P>\r
+\r
+<LI>The __call__ method is invoked when the bound method instance is\r
+called, as in temp(10). It needs to call method1(aninstance, 10).\r
+However, even though self.function is now method1 and self.instance is\r
+aninstance, it can't call self.function(self.instance, args) directly,\r
+because it should work regardless of the number of arguments passed.\r
+(For simplicity, support for keyword arguments has been omitted.)<P>\r
+\r
+</UL>\r
+\r
+<P>In order to be able to support arbitrary argument lists, the\r
+__call__ method first constructs a new argument tuple. Conveniently,\r
+because of the notation *args in __call__'s own argument list, the\r
+arguments to __call__ (except for self) are placed in the tuple args.\r
+To construct the desired argument list, we concatenate a singleton\r
+tuple containing the instance with the args tuple: (self.instance,) +\r
+args. (Note the trailing comma used to construct the singleton\r
+tuple.) In our example, the resulting argument tuple is (aninstance,\r
+10).\r
+\r
+<P>The intrinsic function apply() takes a function and an argument\r
+tuple and calls the function for it. In our example, we are calling\r
+apply(method1, (aninstance, 10)) which is equivalent to calling\r
+method(aninstance, 10).\r
+\r
+<P>From here on, things should come together quite easily. The output\r
+of the example code is something like this:\r
+\r
+<PRE>\r
+calling <function method1 at ae8d8> for <Instance instance at 95ab0> with (10,)\r
+calling <function method2 at ae900> for <Instance instance at 95ab0> with ()\r
+the answer is 10\r
+</PRE>\r
+\r
+<P>That was about the shortest meaningful example that I could come up\r
+with. A real tracing metaclass (for example, <A\r
+HREF="#Trace">Trace.py</A> discussed below) needs to be more\r
+complicated in two dimensions.\r
+\r
+<P>First, it needs to support more advanced Python features such as\r
+class variables, inheritance, __init__ methods, and keyword arguments.\r
+\r
+<P>Second, it needs to provide a more flexible way to handle the\r
+actual tracing information; perhaps it should be possible to write\r
+your own tracing function that gets called, perhaps it should be\r
+possible to enable and disable tracing on a per-class or per-instance\r
+basis, and perhaps a filter so that only interesting calls are traced;\r
+it should also be able to trace the return value of the call (or the\r
+exception it raised if an error occurs). Even the Trace.py example\r
+doesn't support all these features yet.\r
+\r
+<P>\r
+\r
+<HR>\r
+\r
+<H1>Real-life Examples</H1>\r
+\r
+<P>Have a look at some very preliminary examples that I coded up to\r
+teach myself how to write metaclasses:\r
+\r
+<DL>\r
+\r
+<DT><A HREF="Enum.py">Enum.py</A>\r
+\r
+<DD>This (ab)uses the class syntax as an elegant way to define\r
+enumerated types. The resulting classes are never instantiated --\r
+rather, their class attributes are the enumerated values. For\r
+example:\r
+\r
+<PRE>\r
+class Color(Enum):\r
+ red = 1\r
+ green = 2\r
+ blue = 3\r
+print Color.red\r
+</PRE>\r
+\r
+will print the string ``Color.red'', while ``Color.red==1'' is true,\r
+and ``Color.red + 1'' raise a TypeError exception.\r
+\r
+<P>\r
+\r
+<DT><A NAME=Trace></A><A HREF="Trace.py">Trace.py</A>\r
+\r
+<DD>The resulting classes work much like standard\r
+classes, but by setting a special class or instance attribute\r
+__trace_output__ to point to a file, all calls to the class's methods\r
+are traced. It was a bit of a struggle to get this right. This\r
+should probably redone using the generic metaclass below.\r
+\r
+<P>\r
+\r
+<DT><A HREF="Meta.py">Meta.py</A>\r
+\r
+<DD>A generic metaclass. This is an attempt at finding out how much\r
+standard class behavior can be mimicked by a metaclass. The\r
+preliminary answer appears to be that everything's fine as long as the\r
+class (or its clients) don't look at the instance's __class__\r
+attribute, nor at the class's __dict__ attribute. The use of\r
+__getattr__ internally makes the classic implementation of __getattr__\r
+hooks tough; we provide a similar hook _getattr_ instead.\r
+(__setattr__ and __delattr__ are not affected.)\r
+(XXX Hm. Could detect presence of __getattr__ and rename it.)\r
+\r
+<P>\r
+\r
+<DT><A HREF="Eiffel.py">Eiffel.py</A>\r
+\r
+<DD>Uses the above generic metaclass to implement Eiffel style\r
+pre-conditions and post-conditions.\r
+\r
+<P>\r
+\r
+<DT><A HREF="Synch.py">Synch.py</A>\r
+\r
+<DD>Uses the above generic metaclass to implement synchronized\r
+methods.\r
+\r
+<P>\r
+\r
+<DT><A HREF="Simple.py">Simple.py</A>\r
+\r
+<DD>The example module used above.\r
+\r
+<P>\r
+\r
+</DL>\r
+\r
+<P>A pattern seems to be emerging: almost all these uses of\r
+metaclasses (except for Enum, which is probably more cute than useful)\r
+mostly work by placing wrappers around method calls. An obvious\r
+problem with that is that it's not easy to combine the features of\r
+different metaclasses, while this would actually be quite useful: for\r
+example, I wouldn't mind getting a trace from the test run of the\r
+Synch module, and it would be interesting to add preconditions to it\r
+as well. This needs more research. Perhaps a metaclass could be\r
+provided that allows stackable wrappers...\r
+\r
+<P>\r
+\r
+<HR>\r
+\r
+<H2>Things You Could Do With Metaclasses</H2>\r
+\r
+<P>There are lots of things you could do with metaclasses. Most of\r
+these can also be done with creative use of __getattr__, but\r
+metaclasses make it easier to modify the attribute lookup behavior of\r
+classes. Here's a partial list.\r
+\r
+<P>\r
+\r
+<UL>\r
+\r
+<LI>Enforce different inheritance semantics, e.g. automatically call\r
+base class methods when a derived class overrides<P>\r
+\r
+<LI>Implement class methods (e.g. if the first argument is not named\r
+'self')<P>\r
+\r
+<LI>Implement that each instance is initialized with <b>copies</b> of\r
+all class variables<P>\r
+\r
+<LI>Implement a different way to store instance variables (e.g. in a\r
+list kept outside the instance but indexed by the instance's id())<P>\r
+\r
+<LI>Automatically wrap or trap all or certain methods\r
+\r
+<UL>\r
+\r
+<LI>for tracing\r
+\r
+<LI>for precondition and postcondition checking\r
+\r
+<LI>for synchronized methods\r
+\r
+<LI>for automatic value caching\r
+\r
+</UL>\r
+<P>\r
+\r
+<LI>When an attribute is a parameterless function, call it on\r
+reference (to mimic it being an instance variable); same on assignment<P>\r
+\r
+<LI>Instrumentation: see how many times various attributes are used<P>\r
+\r
+<LI>Different semantics for __setattr__ and __getattr__ (e.g. disable\r
+them when they are being used recursively)<P>\r
+\r
+<LI>Abuse class syntax for other things<P>\r
+\r
+<LI>Experiment with automatic type checking<P>\r
+\r
+<LI>Delegation (or acquisition)<P>\r
+\r
+<LI>Dynamic inheritance patterns<P>\r
+\r
+<LI>Automatic caching of methods<P>\r
+\r
+</UL>\r
+\r
+<P>\r
+\r
+<HR>\r
+\r
+<H4>Credits</H4>\r
+\r
+<P>Many thanks to David Ascher and Donald Beaudry for their comments\r
+on earlier draft of this paper. Also thanks to Matt Conway and Tommy\r
+Burnette for putting a seed for the idea of metaclasses in my\r
+mind, nearly three years ago, even though at the time my response was\r
+``you can do that with __getattr__ hooks...'' :-)\r
+\r
+<P>\r
+\r
+<HR>\r
+\r
+</BODY>\r
+\r
+</HTML>\r
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