add method in Python 3

Question

So I'm making my first program in python 3 with operators overloading, and I'm stucked over the add (+) operator.

def __add__(self, newMember):
    if(isinstance(newMember, Queue)):
       tempList=self.myQueue[:] # makes a copy
       tempList.extend(newMember.myQueue)
       return Queue(tempList)

def __str__(self):
    if not self.myQueue:
        string="."
    else:
        string=""
        for x in self.myQueue:
            string=string+str(x)
            if(x<len(self.myQueue)):
                string=string+", "
            else:
                string=string+"."
    return string

basically I'm making a Queue class (I know there's already such one exists), then connecting two Queue objects by typing c=c1+c2. But when I print (c), it messed up the "," and the ".". Can't get what's wrong. Any help?

Shouldn't you do tempList = Queue(self.myQueue) or something like that? I don't know much about Python but if you just assign self.myQueue to tempList then maybe tempList refers to the same object as self.myQueue... — dreamlax
– dreamlax, Commented Apr 21, 2012 at 13:52
I edited my question. I really did reffered to the same object, so I added [:]. I don't want tempList to be a Queue type but a List type so I can return Queue(list) — Jjang
– Jjang, Commented Apr 21, 2012 at 14:04

Wooble · Accepted Answer · 2012-04-21 14:13:54Z

1

To answer your second question (which should probably be a separate question on SO, rather than editing this one):

if(x<len(self.myQueue)): is checking whether the value of a string is less than an integer length of a list. This doesn't make any sense, and will always be False.

You could rewrite the entire method as:

def __str__(self):
    return ', '.join(str(x) for x in self.myQueue) + '.'

answered Apr 21, 2012 at 14:13

Wooble

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Comments

ch3ka · Accepted Answer · 2012-04-21 13:51:21Z

0

in your code, you set tempList to self.myQueue, then modify it. This changes both Queues. you want to copy myQueue, not share the reference.

with tmplist = queue, both variables point to the same Object. Maybe this will help to understand:

>>> queue = []
>>> tmplist = queue
>>> tmplist.append(1)
>>> tmplist
[1]
>>> queue
[1]
>>> tmplist = queue[:] #make a copy
>>> tmplist.append(1)
>>> tmplist
[1, 1]
>>> queue
[1]

answered Apr 21, 2012 at 13:51

ch3ka

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1 Comment

Jjang Over a year ago

Thanks ! Now c1 really doens't change. But i still can't print perfectly, it keep messing up "," and ".". I edited the question adding my str code

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