ex.py

#!/usr/bin/env python3

def make_container_for(key):
    if isinstance(key, int):
        return [None] * (key+1)
    else:
        return {}

def build(val, path):
    for key in reversed(path):
        cont = make_container_for(key)
        cont[key] = val
        val = cont
    return val

def add(val, path, root=None):
    if root is None:
        result = make_container_for(path[0])
    else:
        result = root
    root = result
    for i, key in enumerate(path[:-1]):
        try:
            root = root[key]
        except KeyError:
            root[key] = build(val, path[i+1:])
            return result
        except IndexError:
            root.extend([None]*(key-len(root)))
            root = root[key]
    try:
        key = path[-1]
        root[key] = val
    except IndexError:
        root.extend([None]*(key-len(root)+1))
        root[key] = val
    return result


if __name__ == '__main__':
    r = add("hello", ('a',))
    r = add("hello", ('b',1,), r)
    r = add("hello", ('b',2,), r)
    print(r)