Programming Python, 3rd Edition

Implementing Stacks

Stacks are a common and straightforward data structure, used in a variety of applications: language processing, graph searches, and so on. 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 whether the stack is empty, fetching the top item without popping it, iterating over a stack’s items, testing for item membership, and so on.

In Python, a simple list is often adequate for implementing a stack: because we can change lists in place, we can add and delete items from either the beginning (left) or the end (right). Table 20-1 summarizes various built-in operations available for implementing stack-like behavior with Python lists, depending on whether the stack “top” is the first or the last node in the list. In this table, the string 'c' is the top item on the stack.

Table 20-1. 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	x = stack[-1]; del stack[-1]	x = stack[0]; del stack[:1]	x = stack[0]; stack[:1] = []

Other coding schemes are possible as well. For instance, Python 1.5 introduced a list pop method designed to be used in conjunction with append to implement ...