Nested Python class needs to access variable in enclosing class

Question

I\'ve seen a few \"solutions\" to this, but the solution every time seems to be \"Don\'t use nested classes, define the classes outside and then use them normally\". I don\'

Answer 1

Despite my "bit patronizing" comment (fair play to call it that!), there are actually ways to achieve what you want: a different avenue of inheritance. A couple:

1. Write a decorator that introspects a class just after it's declared, finds inner classes, and copies attributes from the outer class into them.

2. Do the same thing with a metaclass.

Here's the decorator approach, since it's the most straightforward:

def matryoshka(cls):

    # get types of classes
    class classtypes:
        pass
    classtypes = (type, type(classtypes))

    # get names of all public names in outer class
    directory = [n for n in dir(cls) if not n.startswith("_")]

    # get names of all non-callable attributes of outer class
    attributes = [n for n in directory if not callable(getattr(cls, n))]

    # get names of all inner classes
    innerclasses = [n for n in directory if isinstance(getattr(cls, n), classtypes)]

    # copy attributes from outer to inner classes (don't overwrite)
    for c in innerclasses:
        c = getattr(cls, c)
        for a in attributes:
            if not hasattr(c, a):
                setattr(c, a, getattr(cls, a))

    return cls

Here is a simple example of its use:

@matryoshka
class outer(object):

    answer = 42

    class inner(object):

        def __call__(self):
            print self.answer

outer.inner()()   # 42

However, I can't help but think some of the ideas suggested in other answers would serve you better.