Daily Archives: 12 February, 2008

Nicolas: Your metaclass example is a good example of when not to use metaclasses. I wouldn’t be surprised if it is executed slightly different to how you expect. Let’s look at how Foo is evaluated, starting with what’s written:

class Foo:
    __metaclass__ = FooMeta

This is equivalent to the following assignment:

Foo = FooMeta('Foo', (), {...})

As FooMeta has an __new__() method, the attempt to instantiate FooMeta will result in it being called. As the return value of __new__() is not a FooMeta instance, there is no attempt to call FooMeta.__init__(). So we could further simplify the code to:

Foo = {
    'linux2': LinuxFoo,
    'win32': WindowsFoo,
}.get(PLATFORM, None)
if not Foo:
    # XXX: this should _really_ raise something other than Exception
    raise Exception, 'Platform not supported'

So the factory function is gone completely here, and it is clear that the decision about which class to use is being made at module import time rather than class instantiation time.

Now this isn’t to say that metaclasses are useless. In both implementations, the code responsible for selecting the class has knowledge of all implementations. To add a new implementation (e.g. for Solaris or MacOS X), the factory function needs to be updated. A better solution would be to provide a way for new implementations to register themselves with the factory. A metaclass could be used to make the registration automatic:

class FooMeta(type):
    def __init__(self, name, bases, attrs):
        cls = super(FooMeta, self).__init__(name, bases, attrs)
        if cls.platform is not None:
            register_foo_implementation(klass.platform, cls)
        return cls

class Foo:
    __metaclass__ = FooMeta
    platform = None
    ...

class LinuxFoo(Foo):
    platform = 'linux2'

Now the simple act of defining a SolarisFoo class would be enough to have it registered and ready to use.