Creating python objects of same type from nested class

Question

My List class has a nested _Node class within it. With these 2 classes, I can:

Initialize an object of type List within List.
Initialize an object of type _Node within List.
Initialize an object of type List within _Node.

I can't, however, initialize a _Node within a _Node. The last line of the code is below causes NameError: global name '_Node' is not defined

class List:
    def __init__(self):
        self.root = None
        self.brother = None

    def set_root(self):
        self.root = self._Node()

    def set_brother(self):
        self.brother = List()

    def set_root_next(self):
        self.root.set_next()

    class _Node:
        def __init__(self, next_node=None, data=None):
            self.next_node = next_node
            self.data = data

        def set_next(self):
            self.next_node = _Node()

if __name__ == '__main__':
    x = List()
    x.set_root()
    x.set_brother()
    x.set_root_next()

How do I solve this? Making the _Node class unnested works but I am planning to have many types of list in the same file. All accompanying nodes will inherit from a single abstract class so keeping this structure is important.

Where do you initialize the _Node class? It might simply be out of scope. — Piotr Kamoda
– Piotr Kamoda, Commented Feb 27, 2017 at 15:40
To be honest, I would rather use None as the default value for the next node. It makes a cheap and easy way to check if there is a successor. Also, do not use getters/setters, prefer properties. — farsil
– farsil, Commented Feb 27, 2017 at 15:45
@everyone I genuinely appreciate the code quality comments but this was just quickly typed code to illustrate my problem. My actual code looks nothing like this :) — LismUK
– LismUK, Commented Feb 27, 2017 at 15:57

farghal · Accepted Answer · 2017-02-27 15:41:28Z

2

Try using self.__class__() instead

class List:
    def __init__(self):
        self.root = None
        self.brother = None

    def set_root(self):
        self.root = self._Node()

    def set_brother(self):
        self.brother = List()

    def set_root_next(self):
        self.root.set_next()

    class _Node:
        def __init__(self, next_node=None, data=None):
            self.next_node = next_node
            self.data = data

        def set_next(self):
            self.next_node = self.__class__()

if __name__ == '__main__':
     x = List()
     x.set_root()
     x.set_brother()
     x.set_root_next()

answered Feb 27, 2017 at 15:41

farghal

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1 Comment

LismUK Over a year ago

Thanks! Nice and simple, but d'you (or does anyone) know why my code is wrong? As I understand it the scope of the inner class should still allow itself to be instantiated, just like the outer class does.

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