Is there a way to emulate the __prepare__ special method of a Python 3-metaclass in Python 2.5?

Question

In my project I have to stick to Python 2.5 (Google App Engine). Somewhere in the application (actually a framework), I have to keep track which variables are defined and in which order they are defined, in other words I would like to intercept whenever an assignment operator is processed.

Using Python 3, I would define a metaclass M with a __prepare__ method that returns an intelligent dictionary that keeps track of when it is accessed. Then I just have to execute everything inside a class statement with metaclass M.

Is there any way to emulate this in Python 2.5?

EXAMPLE of what I would like to achieve

With the metaclass approach of Python 3, I could implement variables that work like references, for example M could be so that

# y is a callable
class C(metaclass=M):
    x = ref(y)
    x = 1

would be equivalent (up to the creation of C) with y(1), i.e. the first assignment to a variable in C's dictionary by a black-box ref function creates this variable. Further assignments simply call the parameter of the ref function.

Answer 1

You can wrap the variables you populate your classes with with a wrapper that internally keeps a counter and assigns an increasing value. The wrapper may be subclassed for tagging or to add behaviour to the variables. You would use the variable value to order them and a regular Python 2 metaclass to intercept the class creation.

Django is one of the projects that uses this technique to remember the order in which members are defined on a model class. You can take a look at where the fields are created; it is then used in the implementation of the __cmp__ method to keep the fields in the order of creation.