Programming Python, Second Edition

Implementing Stacks

Stacks are a common and straightforward data structure, used in a variety of applications: language processing, graph searches, etc. In short, stacks are a last-in-first-out collection of objects: the last item added to the collection is always the next one to be removed. Clients use stacks by:

• Pushing items onto the top

• Popping items off the top

Depending on client requirements, there may also be tools for such tasks as testing if the stack is empty, fetching the top item without popping it, iterating over a stack’s items, testing for item membership, etc.

In Python, a simple list is often adequate for implementing a stack: because we can change lists in place, we can either add and delete items from the front (left) or end (right). Table 17-1 summarizes various built-in operations available for implementing stack-like behavior with Python lists, depending on whether the stack “top” is the first or last node in the list. In this table, string 'c' is the top item on the stack.

Table 17-1. Stacks as Lists

Operation	Top is end-of-list	Top is front-of-list	Top is front-of-list
New	stack=['a','b','c']	stack=['c','b','a']	stack=['c','b','a']
Push	stack.append('d')	stack.insert(0,'d')	stack[0:0] = ['d']
Pop[a]	X = stack[-1]; del stack[-1]	x = stack[0]; del stack[:1]	x = stack[0]; stack[:1] = []
[a] In fact, Python 1.5 introduced a list pop method designed to be used in conjunction with append to implement stacks: to push ...