- 5.1 Introduction
- 5.2 Lists
- 5.3 Tuples
- 5.4 Unpacking Sequences
- 5.5 Sequence Slicing
- 5.6 del Statement
- 5.7 Passing Lists to Functions
- 5.8 Sorting Lists
- 5.9 Searching Sequences
- 5.10 Other List Methods
- 5.11 Simulating Stacks with Lists
- 5.12 List Comprehensions
- 5.13 Generator Expressions
- 5.14 Filter, Map and Reduce
- 5.15 Other Sequence Processing Functions
- 5.16 Two-Dimensional Lists
- 5.17 Intro to Data Science: Simulation and Static Visualizations
- 5.18 Wrap-Up
This chapter is from the book
This chapter is from the book
This chapter is from the book
5.11 Simulating Stacks with Lists
The preceding chapter introduced the function-call stack. Python does not have a built-in stack type, but you can think of a stack as a constrained list. You push using list method append, which adds a new element to the end of the list. You pop using list method pop with no arguments, which removes and returns the item at the end of the list.
Let’s create an empty list called stack, push (append) two strings onto it, then pop the strings to confirm they’re retrieved in last-in, first-out (LIFO) order:
In [1]: stack = []
In [2]: stack.append('red')
In [3]: stack
Out[3]: ['red']
In [4]: stack.append('green')
In [5]: stack
Out[5]: ['red', 'green']
In [6]: stack.pop()
Out[6]: 'green'
In [7]: stack
Out[7]: ['red']
In [8]: stack.pop()
Out[8]: 'red'
In [9]: stack
Out[9]: []
In [10]: stack.pop()
-------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-10-50ea7ec13fbe> in <module>()
----> 1 stack.pop()
IndexError: pop from empty list
For each pop snippet, the value that pop removes and returns is displayed. Popping from an empty stack causes an IndexError, just like accessing a nonexistent list element with []. To prevent an IndexError, ensure that len(stack) is greater than 0 before calling pop. You can run out of memory if you keep pushing items faster than you pop them.
You also can use a list to simulate another popular collection called a queue in which you insert at the back and delete from the front. Items are retrieved from queues in first-in, first-out (FIFO) order.