Is anyone using meta-meta-classes / meta-meta-meta-classes in Python/ other languages?

Question

I recently discovered metaclasses in python.

Basically a metaclass in python is a class that creates a class. There are many useful reasons why you would want to do

Answer 1

During the History of Programming Languages conference in 2007, Simon Peyton Jones commented that Haskell allows meta programming using Type Classes, but that its really turtles all the way down. You can meta-meta-meta-meta etc program in Haskell, but that he's never heard of anyone using more than 3 levels of indirection.

Guy Steele pointed out that its the same thing in Lisp and Scheme. You can do meta-programming using backticks and evals (you can think of a backtick as a Python lambda, kinda), but he's never seen more than 3 backticks used.

Presumably they have seen more code than you or I ever has, so its only a slight exaggeration to say that no-one has ever gone beyond 3 levels of meta.

If you think about it, most people don't ever use meta-programming, and two levels is pretty hard to wrap your head around. I would guess that three is nearly impossible, and the that last guy to try four ended up in an asylum.

Answer 2

This reminds me of the eternal quest some people seem to be on to make a "generic implementation of a pattern." Like a factory that can create any object (including another factory), or a general-purpose dependency injection framework that is far more complex to manage than simply writing code that actually does something.

I had to deal with people intent on abstraction to the point of navel-gazing when I was managing the Zend Framework project. I turned down a bunch of proposals to create components that didn't do anything, they were just magical implementations of GoF patterns, as though the pattern were a goal in itself, instead of a means to a goal.

There's a point of diminishing returns for abstraction. Some abstraction is great, but eventually you need to write code that does something useful.

Otherwise it's just turtles all the way down.

Answer 3

The class system in Smalltalk is an interesting one to study. In Smalltalk, everything is an object and every object has a class. This doesn't imply that the hierarchy goes to infinity. If I remember correctly, it goes something like:

5 -> Integer -> Integer class -> Metaclass -> Metaclass class -> Metaclass -> ... (it loops)

Where '->' denotes "is an instance of".

Answer 4

To answer your question: no.

Feel free to research it further.

Note, however, that you've conflated design patterns (which are just ideas) with code (which is an implementation.)

Good code often reflects a number of interlocking design patterns. There's no easy way for formalize this. The best you can do is a nice picture, well-written docstrings, and method names that reflect the various design patterns.

Also note that a meta-class is a class. That's a loop. There's no higher level of abstractions. At that point, it's just intent. The idea of meta-meta-class doesn't mean much -- it's a meta-class for meta-classes, which is silly but technically possible. It's all just a class, however.

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Edit

"Are classes that create metaclasses really so silly? How does their utility suddenly run out?"

A class that creates a class is fine. That's pretty much it. The fact that the target class is a meta class or an abstract superclass or a concrete class doesn't matter. Metaclasses make classes. They might make other metaclasses, which is weird, but they're still just metaclasses making classes.

The utility "suddenly" runs out because there's no actual thing you need (or can even write) in a metaclass that makes another metaclass. It isn't that it "suddenly" becomes silly. It's that there's nothing useful there.

As I seed, feel free to research it. For example, actually write a metaclass that builds another metaclass. Have fun. There might be something useful there.

The point of OO is to write class definitions that model real-world entities. As such, a metaclass is sometimes handy to define cross-cutting aspects of several related classes. (It's a way to do some Aspect-Oriented Programming.) That's all a metaclass can really do; it's a place to hold a few functions, like __new__(), that aren't proper parts of the class itself.

Answer 5

Since when I first understood metaclasses in Python, I kept wondering "what could be done with a meta-meta class?". This is at least 10 years ago - and now, just a couple months ago, it became clear for me that there is one mechanism in Python class creation that actually involves a "meta-meta" class. And therefore, it is possible to try to imagine some use for that.

To recap object instantiation in Python: Whenever one instantiates an object in Python by "calling" its class with the same syntax used for calling an ordinary function, the class's __new__ and __init__. What "orchestrates" the calling of these methods on the class is exactly the class'metaclass' __call__ method. Usually when one writes a metaclass in Python, either the __new__ or __init__ method of the metaclass is customized.

So, it turns out that by writing a "meta-meta" class one can customize its __call__ method and thus control which parameters are passed and to the metaclass's __new__ and __init__ methods, and if some other code is to be called before of after those. What turns out in the end is that metcalsses themselves are usually hardcoded and one needs just a few, if any, even in very large projects. So any customization that might be done at the "meta meta" call is usually done directly on the metaclass itself.

And them, there are those other less frequent uses for Python metaclasses - one can customize an __add__ method in a metaclass so that the classes they define are "addable", and create a derived class having the two added classes as superclasses. That mechanism is perfectly valid with metaclasses as well - therefore, so just we "have some actual code", follows an example of "meta-meta" class that allows one to compose "metaclasses" for a class just by adding them on class declaration:

class MM(type):
    def __add__(cls, other):
        metacls = cls.__class__
        return metacls(cls.__name__ + other.__name__, (cls, other), {})

class M1(type, metaclass=MM):
    def __new__(metacls, name, bases, namespace):
        namespace["M1"] = "here"
        print("At M1 creation")
        return super().__new__(metacls, name, bases, namespace)

class M2(type, metaclass=MM):
    def __new__(metacls, name, bases, namespace):
        namespace["M2"] = "there"
        print("At M2 creation")
        return super().__new__(metacls, name, bases, namespace)

And we can see that working on the interactive console:

In [22]: class Base(metaclass = M1 + M2): 
    ...:     pass
    ...: 
At M1 creation
At M2 creation

Note that as different metaclasses in Python are usually difficult to combine, this can actually be useful by allowing a user-made metaclass to be combined with a library's or stdlib one, without this one having to be explicitly declared as parent of the former:

In [23]: import abc

In [24]: class Combined(metaclass=M1 + abc.ABCMeta):
    ...:     pass
    ...: 
At M1 creation