stack_array.py

class Stack:
    '''Implements an efficient last-in first-out Abstract Data Type using a Python List'''

    def __init__(self, capacity):
        '''Creates and empty stack with a capacity'''
        self.capacity = capacity
        self.items = [None]*capacity
        self.num_items = 0

    def is_empty(self):
        if self.capacity == 0:
            return False
        return self.num_items == 0
        # '''Returns True if the stack is empty, and False otherwise
        #    MUST have O(1) performance'''

    def is_full(self):
        return self.num_items == self.capacity
        # '''Returns True if the stack is full, and False otherwise
        #    MUST have O(1) performance'''

    def push(self, item):
        if self.is_full():
            raise IndexError
        else:
            self.items[self.num_items] = item
            self.num_items += 1

        # '''If stack is not full, pushes item on stack.
        #    If stack is full when push is attempted, raises IndexError
        #    MUST have O(1) performance'''

    def pop(self):
        if self.is_empty():
            raise IndexError
        else:
            current_item = self.items[self.num_items - 1]
            self.items[self.num_items - 1] = None
            self.num_items -= 1
            return current_item
        # '''If stack is not empty, pops item from stack and returns item.
        #    If stack is empty when pop is attempted, raises IndexError
        #    MUST have O(1) performance'''

    def peek(self):
        if self.is_empty():
            raise IndexError
        else:
            return self.items[self.num_items - 1]
        # '''If stack is not empty, returns next item to be popped (but does not pop the item)
        #    If stack is empty, raises IndexError
        #    MUST have O(1) performance'''

    def size(self):
        return self.num_items
        # '''Returns the number of elements currently in the stack, not the capacity
        #    MUST have O(1) performance'''