Python class advisors

September 8, 2005

Anyone who has played with Zope 3 has probably seen the syntax used to declare what interfaces a particular class implements. It looks something like this:

class Foo:
    implements(IFoo, IBar)
    ...

This leads to the following question: how can a function call inside a class definition's scope affect the resulting class? To understand how this works, a little knowledge of Python metaclasses is needed.

Metaclasses

In Python, classes are instances of metaclasses. For new-style classes, the default metaclass is type (which happens to be its own metaclass). When you create a new class or subclass, you are creating a new instance of the metaclass. The constructor for a metaclass takes three arguments: the class's name, a tuple of the base classes and a dictionary attributes and methods. So the following two definitions of the class C are equivalent:

class C(object):
    a = 42

C = type('C', (object,), {'a': 42})

The metaclass for a particular class can be picked in a number of ways:

A __metaclass__ variable at module or class scope.
Use the same metaclass as the base class.

If no metaclass is specified through either of these means, an "old style" class is created. I won't cover old style classes here.

Now in Python calling a function and creating a new instance look pretty similar. In fact the metaclass machinary doesn't really care. The following two class definitions are also equivalent:

class C:
    __metaclass__ = type

def not_the_metaclass(name, bases, attrs):
    return type(name, bases, attrs)

class C:
    __metaclass__ = not_the_metaclass

So using a function or other callable object as the metaclass allows you to hook into the class creation without affecting the type of the resulting class.

Class Advisors

The tricks performed by the Zope implements() function are wrapped up in the zope.interface.advice module. It does so by making use of the fact that Python programs can inspect their execution stack at runtime.

Walk up the stack to where the scope of the class being defined.
Check to see if a "__metaclass__" variable has been set, which would indicate the that a metaclass has been specified for this particular class already.
Check the module scope for a "__metaclass__" variable.
Define a function advise(name, bases, cdict) that does the following:
- Deduce the metaclass (either what __metaclass__ was set to in the class scope, the module scope, or check base classes).
- Call the metaclass to create the new class.
- Do something to the new class (in the case of Zope, it sets what interfaces the class implements).
Set the "__metaclass__" variable in the class scope to this function.

The actual implementation is a little more complicated to handle the case of registering multiple class advisors for a single class. The actual interface provided is quite simple though:

from zope.interface.advice import addClassAdvisor

def setA():
    def advisor(cls):
        cls.a = 42
        return cls
    addClassAdvisor(advisor)

class C:
    setA()

This simply sets the attribute 'a' on the class after it has been created. Also, since method decorators are implemented as a single function call, they can add a class advisor as a way to perform some extra work on the class or method after the class has been constructed.

Python