Python recursive class reference - how to make a tree

Question

What's wrong with this?

class Cat():
    parent = Cat()

NameError: name 'Cat' is not defined

All I wanted to do was to create a class that could be nested to create a tree of category objects. What's the standard way of doing this?

I will eventually be putting this in Django as a Model and syncing with SQLite3. I am a Python newbie. Sorry...

How exactly do you plan to do this without recursing infinitely, since each `Cat`'s parent is a brand new `Cat`? — TheSoundDefense, Jul 23 '14 at 16:29
Thanks all for your answers. It is a duplicate and answered here: http://stackoverflow.com/a/1020431/1369798 — Alveoli, Aug 01 '14 at 23:34
The short answer is: class Cat(self): parent = ForeignKey('self') — Alveoli, Aug 01 '14 at 23:36

score 3 · Accepted Answer · answered Jul 23 '14 at 16:30

3

You are trying to create a Cat object before the definition of the Cat class is completed.

This version is syntactically legal, but will create an infinite list of cats :

class Cat():
    def __init__(self):
         self.parent = Cat()

What you probably want is something like this:

class Cat():
    def __init__(self, parent=None):
         self.parent = parent

answered Jul 23 '14 at 16:30

Hans Then

10,935
3
32
51

1

This will recurse infinitely, won't it? – TheSoundDefense Jul 23 '14 at 16:30
Yes. The first version will, but the second version is probably what he wants. – Hans Then Jul 23 '14 at 16:33

score 1 · Answer 2 · answered Jul 23 '14 at 16:31

1

class Cat:
    def __init__(self,parent):
        self.parent = parent

Cat(Cat("dog"))

maybe?

answered Jul 23 '14 at 16:31

Joran Beasley

110,522
12
160
179

Daniel Roseman · Answer 3 · 2015-04-15T07:57:49.967

Apart from the problems of infinite recursion, it's a mistake to model something as a plain class and then later think about how you would convert it into a Django model. This is because of the object-relational impedence mismatch: Django models are meant to be stored in the database, and relational dbs never match up perfectly with oop concepts.

Luckily, there exists a very good algorithm for spring hierarchical data, which is MPTT, and a good Django implementation of that, django-mptt.

Python recursive class reference - how to make a tree

3 Answers3