Instance variables in another class (no single inheritance)

Question

I want to know how to use an instance variable (class A) in class B. Here is a small code quote that runs normally:

class Node:
   def __init__(self, value):
      self.value = value
      self.next = None
 
class Stack:
   def __init__(self):
      self.head = Node("head")
      self.size = 0
   def __str__(self):
      cur = self.head.next
      out = ""
      while cur:
         out += str(cur.value) + "->"
         cur = cur.next
      return out[:-3]

I wonder how I can use instance variables (.next and .value) in the class Stack. I think instance variables (.next and .value) can appear and be used only in Node class. As I know, we should use single inheritance and class Stack should change to class Stack(Node). Also, I don't understand what while cur means. Is it the same as while True?

Could you explain this problem for me?

Wolf · Accepted Answer · 2021-08-17 11:47:28Z

2

Question_1 : how I can use instance variables (.next and .value) in the class Stack. I think instance variables (.next and .value) can appear and be used only in Node class. As I know, we should use single inheritance and class Stack should change to class Stack(Node)?

Answer_1 : in def ___init__ we create variable of Node then we can use .next and .value of self.head because this variable is instance of Node class.

Question_2 : Also, I don't understand what while cur means. Is it the same as while True?

Answer_2 : for first Node we set .next = None then when we have multiple nodes we run on nodes until get Node.next == None then None assign to cur and we exit from while-loop when cur will be None.

edited Aug 17, 2021 at 11:47

Wolf

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answered Aug 17, 2021 at 9:46

Mahdi F.

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10 Comments

Wolf Over a year ago

You are welcome. The core of your answer is correct but you should pay more attention to case (Python is case-sensitive).

Mahdi F. Over a year ago

@Wolf, I'll always remember your advice.

Mahdi F. Over a year ago

@jacobdavis, yes and when whil cur is None we exit from while-loop

Mahdi F. Over a year ago

@jacobdavis, Node("head") or Node() or Node(None) you pass "head" , "" , None for self.value of instance of Node class. you can send any but better if you send string in all of your programs send a string is better you keep routine in all your programs.

Mahdi F. Over a year ago

@jacobdavis, no you can not remove it, for the first node we set it None but for other Nodes, we set this to a reference other Nodes, do you know about Linked List in this code we create a stack with Linked List, see these linkes maybe help you: geeksforgeeks.org/linked-list-set-1-introduction, geeksforgeeks.org/stack-in-python

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