Stack operations in Python

A stack has two main operations:

• PUSH: insert a new element at the TOP. Pushing onto a stack that is already full causes overflow.
• POP: remove the element at the TOP. Popping from an empty stack causes underflow.

In Python, a list makes a simple stack, with its right end as the TOP:

• append(element) pushes an element onto the top;
• pop() removes and returns the top element.

Because these work at the end of the list automatically, there is no need to keep a separate TOP variable. And since a list has no fixed size, overflow does not happen, unless the computer runs out of memory.

For example, starting with the stack [1, 2]: push 3 gives [1, 2, 3]; pop removes 3; another pop removes 2; the stack is left as [1].

The book builds a stack of glasses, glassStack, with a set of functions:

• isEmpty(glassStack): returns True if the stack is empty, by checking whether its length is 0. It should be checked before popping, to avoid underflow.
• opPush(glassStack, element): pushes with append().
• opPop(glassStack): pops with pop(), after checking that the stack is not empty.
• size(glassStack): returns the number of elements, using len().
• top(glassStack): returns the top element, glassStack[len(glassStack)-1], without removing it.
• display(glassStack): prints the elements from top to bottom.