attributes in python class

Question

I have problem updating attributes in a python class, I give a small example.

#!/usr/bin/python
# -*- coding: utf-8 -*-
class myClass():
    def __init__(self):
        self.s=0
        self.procedure()
    def procedure(self):#{{{
        s=self.s
        print s
        self.operation()
        print s
    def operation(self):#{{{                                                                                            
        s=self.s
        s+=1
        self.s=s
        print "o:",self.s
c=myClass()

then output

0
o: 1
0

why the last number is 0 and not 1 ?

Michael Hoffman · Accepted Answer · 2011-11-18 20:11:40Z

2

in myClass.procedure(), you create the local name s with a value copied from self.s, which is 0. In myClass.operation(), you set self.s to 1. But you haven't overwritten your previous copy to s in myClass.procedure(), so it is still 0.

answered Nov 18, 2011 at 20:11

Michael Hoffman

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Comments

user · Accepted Answer · 2011-11-18 20:13:02Z

1

Your procedure code should read:

def procedure(self):#{{{
        s=self.s
        print s
        self.operation()
        # the following line is edited
        print self.s

Otherwise, the variable you change (self.s) doesn't print.

answered Nov 18, 2011 at 20:13

user

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Comments

Chris Lacasse · Accepted Answer · 2011-11-18 20:10:36Z

1

You are printing s instead of self.s in the last print.

answered Nov 18, 2011 at 20:10

Chris Lacasse

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2 Comments

glglgl Over a year ago

@JuanPablo But only until self.s gets set to something else.

Michael Hoffman Over a year ago

@JuanPablo No, it's not. Originally s and self.s are two names for the same object. But then you change the object that self.s is the name for without changing the object that s is the name for.

Karl Knechtel · Accepted Answer · 2011-11-18 21:07:13Z

Because s = self.s doesn't make s another name for "whatever you get by checking self.s, always and forever"; it makes s another name for "whatever self.s currently refers to".

The integer 0 is an object. In the first function, you cause self.s and s to both refer to that object, and then cause self.s to refer to something else (the result of the addition) via another s that's local to the other function. The s local to the first function is unchanged.

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