Question about a python class [SOLVED]

Lexion · 2009-10-13 13:00:00

I have a class:

  1 class Node:
  2     __value = ""
  3     __others = []
  4     def __init__(self):
  5         self.__value = ""
  6     def set(self, n):  
  7         self.__value = n
  8     def get(self):
  9         return self.__value
 10     def connect(self, n):
 11         self.__others.append(n)
 12     def add(self, n):
 13         tmp = Node()
 14         tmp.set(n)
 15         self.connect(tmp)
 16     def show(self):
 17         print self.get()
 18         print "-> ",
 19         for i in self.__others:
 20             print i.get()
 21             print "   ",
 22         print ""
 23     def go(self, n):
 24         for i in self.__others:
 25             if n == i.get():
 26                 return i
 27         print "error: no results"

which attempts to be a network of interconnected nodes (each with a dynamic number of connections). The problem is that the __others list seems to be global amoungst all instances of Node.

Is there any way of getting around this?

Last edited by Lexion (2009-10-14 14:33:38)

Trent · 2009-10-13 13:25:08

You can't initialize instance variables in a class directly -- you have to do it in the constructor. That is,

  1 class Node:
  2     __value = ""
  3     __others = []

initializes the variables Node.__others and Node.__value, not attributes of any instance. Your problem is probably that you don't initialize self.__others in the constructor. When you call an append() on self.__others, Python looks in the `self' object for an attribute called __others, and doesn't find it. So it moves up one level and looks in the Node class for such an attribute, finds it, and changes it. This doesn't happen with self.__value, because self.__value is a different object from Node.__value due to the fact that you changed it in the constructor.

OT: is there a particular reason for using `__value' and `__others' instead of just `value' and `others'?

winch · 2009-10-13 20:25:01

To put a name to it __value and __others are class variables.
http://www.ibiblio.org/g2swap/byteofpyt … -vars.html

This probably explains the use of double underscores.
http://docs.python.org/tutorial/classes … -variables

Trent · 2009-10-13 21:29:06

I'm aware of the effects of leading-double-underscore names in Python. I was just wondering why in this particular instance the OP would specifically want this behavior. Unless you want to avoid conflict with an attribute of a superclass, the name-mangling doesn't offer any features of value.

keenerd · 2009-10-14 00:49:33

Just so that you are aware, because this is the most common cause of trouble I see with people learning Python. You are using old style classes, which are in there for reverse compatability. You probably don't mean to use them.

# old style
class Node:
    pass

# new syle
class Node(object):
    pass

Lots of awesome features like attribute properties and decorators only work with new style classes.

To answer your question, you should have the variables in __init__, unless you want them to be global to the class.

Lexion · 2009-10-14 14:33:19

Thanks, I got it.

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#1 2009-10-13 13:00:00

Question about a python class [SOLVED]

#2 2009-10-13 13:25:08

Re: Question about a python class [SOLVED]

#3 2009-10-13 20:25:01

Re: Question about a python class [SOLVED]

#4 2009-10-13 21:29:06

Re: Question about a python class [SOLVED]

#5 2009-10-14 00:49:33

Re: Question about a python class [SOLVED]

#6 2009-10-14 14:33:19

Re: Question about a python class [SOLVED]

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