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+
+<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>
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