Q1] Practicing Object-Oriented Programming

A stack is a data structure in which items are
inserted at one end and must be removed from the same end.  As an
analogy, think of a stack of trays in a cafeteria.  When you add new
trays you push the whole stack down and when you remove a tray the
stack pops up.  Implement a Stack object with the following
methods:

__init__, which creates an empty stack
__str__, which returns a string representing the contents
of the stack
empty, which tests whether the stack is empty
push, which adds an item to the top of the stack
pop, which removes and returns the item at the top of the
stack
size,  which returns the number of items on the stack


You may store the items in your stack using a python list and use the list methods append, pop, remove, etc., to implement your stack methods. You can test your class definition with the following program:

def main():
    s = Stack()                           
    if s.empty():
        print "Stack is empty"
    s.push('a')
    print s
    s.push('b')
    print s
    s.push('c')
    print s
    print s.size()
    print s.pop()
    print s

This program should produce the output:

Stack is empty
Stack: a 
Stack: b a 
Stack: c b a 
3
c
Stack: b a 


Q2] Explain the following object-oriented concepts and cite
specific examples from the previous problem to illustrate your points.

method
instance variable
instance of a class
constructor
accessor
mutator