Python for Everyone: The PreTeXt Interactive Edition

A program in its most basic form takes some input, does some processing, and produces some output. Our elevator conversion program demonstrates a very short but complete program showing all three of these steps.

If we think a bit more about this program, there is the “outside world” and the program. The input and output aspects are where the program interacts with the outside world. Within the program we have code and data to accomplish the task the program is designed to solve.

One way to think about object-oriented programming is that it separates our program into multiple “zones.” Each zone contains some code and data (like a program) and has well defined interactions with the outside world and the other zones within the program.

If we look back at the link extraction application where we used the BeautifulSoup library, we can see a program that is constructed by connecting different objects together to accomplish a task:

import urllib.request, urllib.parse, urllib.error
from bs4 import BeautifulSoup
import ssl

# Ignore SSL certificate errors
ctx = ssl.create_default_context()
ctx.check_hostname = False
ctx.verify_mode = ssl.CERT_NONE

url = "https://www.nytimes.com/"
html = urllib.request.urlopen(url, context=ctx).read()
soup = BeautifulSoup(html, 'html.parser')

# Retrieve all of the anchor tags
tags = soup('a')
for tag in tags:
    print(tag.get('href', None))

We pass the url into urllib to retrieve the data from the web. The urllib library uses the socket library to make the actual network connection to retrieve the data. We take the string that urllib returns and hand it to BeautifulSoup for parsing. BeautifulSoup makes use of the object html.parser^[https://docs.python.org/3/library/html.parser.html¹] and returns an object. We call the tags() method on the returned object that returns a dictionary of tag objects. We loop through the tags and call the get() method for each tag to print out the href attribute.

We can draw a picture of this program and how the objects work together.

The key here is not to understand perfectly how this program works but to see how we build a network of interacting objects and orchestrate the movement of information between the objects to create a program. It is also important to note that when you looked at that program several chapters back, you could fully understand what was going on in the program without even realizing that the program was “orchestrating the movement of data between objects.” It was just lines of code that got the job done.