Type the code into IDLE, save the program, and run it.

The code you’ve been left goes to the prices page on the Beans’R’Us website to get the current price of coffee beans. But instead of giving just the cost, it gives you all of the HTML text used to create the web page itself.

Let’s work out what each of the following substring specifications mean. Imagine the variable s is set to the string below. Your job is to determine what each of the substrings provide.

In general, if you specify a substring using s[a:b], then:

Imagine the variable s is set to the string below. Your job was to determine what each of the substrings provide.

In general, if you specify a substring using s[a:b], then:

You need to update the program to extract the price starting at the 235th character of the string. The price is four characters long. Store the price substring in a variable called price. Write the new version of the program here:

You needed to update the program to extract the price starting at the 235th character of the string. The price is four characters long and stored in a variable called price.

Type the code into IDLE, save the program (choose a name for the program that is meaningful to you), and run it.

That looks a lot cleaner. Now, instead of displaying the entire HTML text of the web page, you’ve cut it down to just the piece of the string (the substring) that you need.

This week’s interview: We ask the String what it’s like being the world’s most eligible datatype.

Head First: String, it’s so good of you to find the time to speak to us.

String: Please, the honor is mine. Sit. Sit. Make yourself at home. Did you eat yet?

Head First: I’m fine, thank you. String, where should I begin? You are known the world over for your work. In your time you’ve carried the works of Shakespeare, Goethe...

String: Dan Brown.

Head First: ...all the great works of literature. And even mundane things like names and addresses. Tell me, how did you become so popular?

String: It’s a question of character. Well, characters. See, before I existed, computer systems used to record text one character at a time.

Head First: That must have been rather inconvenient.

String: Inconvenient? It was a royal pain in the tuchis.

Head First: Quite.

String: Without me, handling text was like riding a pedal cycle without a saddle.

Head First: In what way?

String: It was possible to get somewhere, but the journey was kind of stressful.

Head First: You simplify things.

String: Certainly. I simplify. Instead of keeping track of a hundred, or a thousand, or a million letters, you just need keep an eye on one thing. Me!

Head First: That’s a good point.

String: I like to think of myself as a front. An agent, you might say, for all the characters I work with.

Head First: People deal with you, so they don’t have to deal with individual characters in memory.

String: Exactly. I’m an organizer. I keep an eye on the day to day business of the letters. If I need to be shorter or longer, I arrange for the characters to be made available.

Head First: Tell me about your substrings.

String: Ah, my substrings. Like chips off the old block. That a humble datatype should be so blessed!

Head First: A tissue?

String: Bless you. <blows nose>. Those boys are so close to me. Here’s a photo. Can you see the resemblance?

Head First: Why he looks just like...

String: Ah, you guessed! Yes, my character sequence from 137 to 149. Exactly. Just like his old man. But shorter. Little more hair.

Head First: Your substrings are strings as well.

String: Certainly. Strings just like me. And they, I hope, should one day be able to produce their own substrings as well.

Head First: Yet some people are confused by your indexing.

String: What can I say? I started with nothing!

Head First: String, thank you.

String: A pleasure. Are you sure you ate?

This time when you run it, this happens...

The program is no longer displaying a price. So what happened?

These are some of the many built-in string methods that come with Python. Match each method to what it does. We’ve done one for you already.

Which of the above methods do you need to use to locate the price substring within the Beans’R’Us web page?

_____________________________________________

These are some of the many built-in string methods that come with Python. You were to match each method to what it does.

Which of the above methods do you need to use to locate the price substring within the Beans’R’Us web page?

You need to update your price-grabbing program so that it extracts the four-character substring that follows the occurence of the “>$” characters. Write the new version of your code in the space provided.

Hints: Don’t forget that the find() method finds the starting position of a substring. Once you’ve found “>$”, use Python’s addition operator to calculate where in the string you want to extract the substring. The addition operator is the “+” symbol.

You needed to update your price-grabbing program so that it extracts the four-character substring that follows the occurence of the “>$” characters.

Hints: Don’t forget that the find() method finds the starting position of a substring. Once you’ve found “>$”, use Python’s addition operator to calculate where in the string you want to extract the substring. The addition operator is the “+” symbol.

OK, so your program should now be able to find the price, no matter where it appears in the page.

It works! By adding very little extra code, you have made the program much smarter and more useful.

The program code to add the feature is sitting on the fridge door. Your job is to arrange the magnets so that the program loops until the price falls to $4.74 or lower.

The program code to add the feature was sitting on the fridge door. You were asked to arrange the magnets so that the program loops until the price falls to $4.74 or lower.

Enter the new version of the program code into an IDLE edit window and run it.

It looks like something’s gone wrong with the program. What does TypeError mean? What’s happened?

This should be a pretty quick fix. If you use the float() function to convert the price substring, you should then be able to compare the price to 4.74:

That’s much better. Your program now waits patiently until the price falls to the right level and only then does it tell the CEO that the time is right to buy a fresh batch of coffee beans.

These are some of the functions provided by Python’s built-in time library:

You need to use one of these functions to help you fix your code.

But which one? Draw a circle around the function you think you might need.

With the appropriate function identified, amend the code to control how often the request for the web page is sent to the server. The Beans’R’Us webmaster has been in touch to say that their web-based pricing information is updated every 15 minutes. Fill in the blanks in the code as indicated by the dashed lines.

Hints: 15 minutes equates to 15 multiplied by 60 seconds, which is 900 seconds. Also: to use the functionality provided by a library, remember to import it first.

import urllib.request

______________________

price = 99.99
while price > 4.74:
    _____________________
    page = urllib.request.urlopen("http://www.beans-r-us.appspot.com/prices-loyalty.html")
    text = page.read().decode("utf8")
    where = text.find('>$')
    start_of_price = where + 2
    end_of_price = start_of_price + 4
    price = float(text[start_of_price:end_of_price])
print ("Buy!")

These are some of the functions provided by Python’s built-in time library:

You need to use one of these functions to help you fix your code.

But which one? You were to draw a circle around the function you thought you might need.

With the appropriate function identified, you were to amend the code to control how often the request for the web page is sent to the server. The Beans’R’Us webmaster has been in touch to say that their web-based pricing information is updated every 15 minutes.You were to fill in the blanks in the code as indicated by the dashed lines.

Hints: 15 minutes equates to 15 multiplied by 60 seconds, which is 900 seconds. Also: to use the functionality provided by a library, remember to import it first.