Question

I am a C++ coder. Recently started with Python. I was having a look at a simple Linked List implementation in Python. I am bit confused here. Not only here but also in Tree implementation and so on with the same problem.

class Element contains data and pointer to next node. Perfect no problem. However in class LinkedList I can see self.tail.next=e, now next is a variable of Element class even if it is public than also an object of Element class has to access it. Here how can we write something like self.tail.next = e as tail is just a variable of LinkedList class and is not an object of Element class. I am confused.

class Element:
        def __init__(self,x):
                self.data=x
                self.next=None


class LinkedList:
        def __init__(self):
                self.head=None
                self.tail=None

        def append(self,x):
                # create a new Element
                e = Element(x)
                # special case: list is empty
                if self.head==None:
                        self.head=e
                        self.tail=e
                else:
                        # keep head the same
                        self.tail.next=e
                        self.tail=e

Answer 1

Python works with references. Everything is always passed by reference, the values are always shared via references (unless explicitly copied).

Assigning an object means assigning the reference to that object. This way self.tail.next = e means: self.tail is expecting to refer to the object of the Element class. The object has the .next attribute. The self.tail.next = e means that the last element of the non-empty list is going to point to the just appended new element. Then the self.tail = e means that the reference to the last element is moved to the just appended last element.

Any variable in Python is just a reference variable with the given name. It is automatically dereferenced. Because of that it may look strangely to those familiar with classical compiled languages, like C++.

I am not sure if you can display the articles at Expert Exchange without creating the account. If yes, have a look at http://www.experts-exchange.com/Programming/Languages/Scripting/Python/A_6589-Python-basics-illustrated-part-2.html and namely the http://www.experts-exchange.com/Programming/Languages/Scripting/Python/A_7109-Python-basics-illustrated-part-3.html for the images that explain the problem.