16.4. Starting with programs

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.

usf = input('Enter the US Floor Number: ') wf = int(usf) - 1 print('Non-US Floor Number is',wf)

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.html] 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.

Q-3: Which of the following is true about the following code?

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')
print(soup('a'))

• BeautifulSoup creates and returns an object to soup
• BeautifulSoup makes use of the object 'html.parser' and returns an object.
• The following program is sequential and not object oriented.
• The program may look sequential but it also calls on other classes to return objects.
• The following program is an example of procedural programming.
• The program inherits functions from other classes with the import statements instead of using procedures.
• The program will give a 'NameError' as function BeautifulSoup is called before its defined.
• BeautifulSoup is imported to the program.

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.

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