Chapter 3. Silly Sentence Generator 3000: creating interactive programs

In this chapter, you’ll see how you can use Python to

• Create a welcome message for a game
• Add notes to your code
• Ask users to input (or type in) information and save it using variables
• Join strings
• Display information back to the user based on that information

Visit a website, start up a game system, or open a mobile application, and it will probably ask you to enter a name and email address and create a password. These are all computer programs, and once you’re logged in, they may display special messages at the top of the screen saying things like “Welcome, Aaron” (or whatever your name is). Some programs are very sophisticated, remembering the games you’ve played, the badges you’ve earned, the balance in your account, or the products you’ve viewed.

iTunes, Netflix, Facebook, and Gmail are all sites that use computer programs that ask you for information, save information, and interact with you based on that information. In this chapter, you’ll see how to do this with Python by creating a ridiculously fun word game called Silly Sentence Generator 3000.

Creating a welcome message

In Silly Sentence Generator 3000, the game player (that’ll be you) is asked to enter words such as nouns, verbs, adjectives, and so on. You’ll store the words as variables and then use them to create ridiculous, nonsensical sentences.^[1] Figure 3.1 shows an example of what the finished program looks like.

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This is similar to the game Mad Libs, if you’ve ever played it.

Figure 3.1. Silly Sentence Generator 3000 asks the user to enter their name and some words, and then it creates a silly sentence from those words.

Think about the program like a machine that takes a set of inputs and then creates an output. You’re going to put together the machine by creating the instructions that drive it. Conceptually, this “machine” might look something like figure 3.2. Change the words you put in, and you’ll get a completely different result. That’s part of what makes games so much fun!

Figure 3.2. An interactive game lets you put in information, and then it creates an output.

Let’s see how to create this game. Once you create it, you can change it and add to it however you like.

Starting a new program

If you open a game, one of the first things you see is a main menu or title screen. Let’s use what you know about displaying text on the screen to make your program display a title for your game. You start by opening IDLE and creating a new program. Open IDLE for Python 3 by clicking the Menu button and selecting Programming > Python 3 on your Raspberry Pi’s desktop (see figure 3.3).

Figure 3.3. Select Menu-->Programming-->Python 3 to open the Python Shell on your Raspberry Pi.

Give your Raspberry Pi a few seconds to open IDLE. After IDLE opens, you’ll see the Python Shell (see figure 3.4).

Figure 3.4. The Python Shell

Press Ctrl-N or choose File > New Window to open the IDLE text editor. You’ll see a blank window, ready for you to start typing in your program (see figure 3.5).

Figure 3.5. The IDLE text editor is where you can type in your Python program. You can also edit, save, and run programs using the menu options.

Using the print function you learned about in chapter 2, let’s make a title screen:

print("*" * 48)
print("* Welcome to the Silly Sentence Generator 3000 *")
print("*" * 48)

Excellent. Feel free to elaborate on the welcome message and the artwork with different characters. Before you go much further, you should save the program.

Saving the program

Save the program by selecting File > Save or pressing Ctrl-S. This will open a window asking where you want to save the program and what to name it. Let’s name it SillySentence (see figure 3.6). By default, IDLE saves your file to your /home/pi folder. Let’s use that folder.

Figure 3.6. Save your file as SillySentence. This stores the file on your Raspberry Pi in your /home/pi folder so you can run the program and make changes to it.

Click Save, and the file will be saved as SillySentence.py (the .py file extension is automatically appended by IDLE). After you save the file, the title at the top of the text editor window will show the filename and file location, as you can see in figure 3.7.

Figure 3.7. The first three lines of your program use the print function to create a welcome message for the Silly Sentence Generator 3000 program.

Guess the output. What do you think you’ll get when you run the program?

Let’s try it. Click Run > Run Module (or press the keyboard shortcut F5). Python will read each line of your program and execute the commands. The commands print a line of * characters, the welcome message, and another line of * characters to the screen (see figure 3.8).

Figure 3.8. Running the program SillySentence.py displays a welcome message on the screen.

Excellent! Now you have a proper welcome message for your game. The next thing you need to do is gather some input from your game player. Some games use button presses, but you’ll use the keyboard for this game.

pi@raspberrypi ~ $

pi@raspberrypi ~ $ python3 SillySentence.py

Adding notes in your code

Imagine a comic book without words. You’d have a hard time understanding what was happening from just the pictures. Maybe you could figure it out if you studied the comic long enough, but words are important for understanding a story. Lines of code can be like a comic book without words: you know something is happening, but you might not be able to tell what without guessing.

That’s why programmers invented the idea of adding comments. Comments are notes in the code that explain what’s happening. They’re as much for you as for other people who may read your code. You can use comments to explain why you wrote the program and how parts of the program work.

Using hashtags for comments

You add a comment by starting the line with a hashtag (#) and a space and then typing in your comment text. Let’s add comments to the beginning of Silly Sentence Generator 3000 to explain the program’s title, its purpose, and who wrote it.

Listing 3.1. Adding notes to your program

Comments are helpful to the humans reading the code. But Python ignores comments when it runs your program. You can check this by saving your program and running it again; you’ll see that you get the same result as before.

Python’s creator, Guido van Rossum, said that code is read more often than it’s written.^[2] Readability is an extremely important part of programming and is a guiding principle in the style of Python programs. Comments are an important way to keep your code easy to read and understand.

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Check out the resource PEP 8—the Style Guide for Python, written by Python’s creators: http://legacy.python.org/dev/peps/pep-0008. A wonderful section called “A Foolish Consistency Is the Hobgoblin of Little Minds” talks about the importance of readable code.

Comments are your new friend, and they will make your code easy to read. You’ll keep using them to add notes to your code as you collect information from your game player (or user) and create a silly sentence.

Getting and storing information

To gather input from users, you can use the input function. Let’s add a line of code in your program that will ask the user for their information and store that information in a variable.

Listing 3.2. Gathering input from the player

When you use the input function, it displays a prompt and awaits the user’s reply. After the user enters something and presses Enter, the information is stored in the variable on the left side of the equals sign.

In the IDLE editor, input shows up in purple highlighting, indicating that it’s the name of a function in Python. Let’s look closely at the input function to see how it works (see figure 3.9).

Figure 3.9. The input function displays a prompt to the user. The prompt “Please enter your name:” tells the user what you want them to type in or enter. In this case, you’re asking for their name.

On the right side of the equals sign, the input function is called, and you open a set of parentheses. You can give the input function a string that acts as the prompt. This is the message that is displayed on the screen and that says to the user, “Hey you, please type something in”—only more nicely! Make sure your string starts and ends with quotation marks ("").

Run the program by pressing F5 or selecting Run > Run Module. The program displays the welcome message and then an input prompt with a blinking cursor. Python is waiting for your input: it needs you to type something in and press Enter.

On the left side of the equals sign is the name of a variable in which the information will be stored. When you type something in and press Enter, the value of what you typed is stored in the variable player_name as a string.

Python 2.X

The input function was previously raw_input in Python 2.X.

Joining strings

As in other apps and websites, you want the user to feel welcome, so let’s use their name and give them a proper greeting. A nice message to display on the screen might be

"Hello, Ryan! Let's make a silly sentence!"

To create a personal feel, you’ll create just such a message that joins the user’s name with some words welcoming them. You use the plus (+) symbol to join strings:

message =  "Hello, " + player_name + "!  Let's make a silly sentence!"

If player_name equals “Melissa”, the message is equal to

"Hello, Melissa!  Let's make a silly sentence!"

Add this to your program, and display the message to the screen using print.

Listing 3.3. Using + to join strings

The program has the user input their name, which is stored in the variable player_name. On the next line, a message is made by joining strings. The message is displayed on the screen to create a personalized start for the game.

"jOHn".capitalize()

"RABBIT".lower()

"king Arthur".upper()

"John Cleese".lower()

>>> TestAnswers = "TTTFFFTTTFTFFFFTTTFFTT"
>>> TestAnswers.count("T")
12

Let’s go further and add more inputs.

Using more than one input

You have a wonderful start to your game. Now you need to gather multiple inputs from the player. Let’s start by asking the player for a noun—the name of a famous person:

famous_person = input("Enter the name of a famous person: ")

Next, you should get a few more words:

adjective1 = input("Enter an adjective: ")
adjective2 = input("Enter another adjective: ")
verb = input("Enter a verb ending in –ING: ")

With these multiple inputs, your code should now look like the following listing.

Listing 3.4. Collecting multiple items from the player

You use the input function multiple times to collect a set of words from the user. Each word is stored in a variable on the left side of the equals sign. Try to use names for variables that make sense; it’ll be easier to remember what you stored in them later.

Building the sentence

Now let’s create the sentence for your Silly Sentence Generator 3000 by joining the words using +:

silly_sentence = ("The " + adjective1 + " " + player_name + " is " +
                 verb + " the " + adjective2 + " " + famous_person)

Let’s take a closer look at this line of code in figure 3.10 to see what’s happening.

Figure 3.10. silly-sentence is created by joining a set of strings. The strings are a combination of strings you enter with quotation marks around them and strings collected from the game player that are stored in variables. The parentheses are needed because the code is too long to fit on a single line.

On the right side of the equals sign, the parentheses enclose the strings that are being joined to create a sentence. They’re joined (or concatenated) using the + operator. Because the line is so long, you can use a set of parentheses to break it over two lines. Python recommends limiting all lines to no longer than 79 characters so the code can be easily read. Looking at the left side of the equals sign, you’ll see that the resulting string is stored in a variable named silly_sentence.

What’s especially awesome is that this code will create a different sentence each time a user enters different words. Because you used variables and the variables are storing the input from the user, it’s truly a Silly Sentence Generator!

Troubleshooting

When typing code, it’s easy to make mistakes, called bugs. Boo to bugs. To track them down and fix them, you debug your code. Yay for debugging. You may forget to close a set of quotation marks, you may leave out a parenthesis, or you may misspell a word. Let’s look at some common errors you might make and how to fix them.

In the last section, you used the + to join strings and variables that were storing strings. Look at this code, which has an error:

silly_sentence = ("The  + adjective1 + " " + player_name + " is " +
                 verb + " the " + adjective2 + " " + famous_person)

Do you see the problem? The first string ("The) is missing the closing quotation mark ("The "). If you were to run this program, Python would output an error (see figure 3.11). Add the closing quotation mark to the string that is missing it, and then save your program and run it again.

Figure 3.11. If you forget to close a set of quotation marks around a string, you’ll receive an error from Python when you try to run your program. Python will highlight in red the line with the error. Check each of the strings to find and fix the error.

Another common error you might make is to misspell the name of a variable or use different capitalization. Here’s the same line of code, but this time there is a misspelled variable and one variable with incorrect capitalization. Can you spot them?

silly_sentence = ("The " + adjectve1 + " " + player_name + " is " +
                 verb + " the " + adjective2 + " " + Famous_person)

The first one is adjectve1, which should be adjective1 (the i is missing). The second error is Famous_person, which should be famous_person (the F should be lowercase). The error you’ll see if you run this program is shown in figure 3.12.

Figure 3.12. A common mistake in programming is to misspell the name of a variable or use incorrect capitalization. The error displayed says there is a problem on line 25 of the program. The type of error is NameError: name 'adjectve1 ' is not defined.

Tip

The spelling and capitalization of a variable must always be the same. If you call a variable my_number and then later type my_nomber or My_number, Python will give you an error.

Correct the error by fixing the spelling of adjectve1 so it’s adjective1. After fixing it, you’ll still receive an error, but this time because of the capitalization of Famous_person (NameError: name 'Famous_person' is not defined). Change the capitalization of Famous_person to famous_person. Once you’ve made the corrections, save the program and run it again.

You’ve debugged your program. Superb job!

Completing the program: displaying the silly sentence

You’ve made your silly sentence, and you want Python to show it to the player. Use the print statement to print it out, but like your welcome message, let’s add some pizzazz to it!

print("*" * 48)
print(silly_sentence)
print("*" * 48)

Guess what it does? It prints a row of * characters (asterisks) across the screen 48 times. Then it displays the sentence and prints another row of * symbols 48 times. Try other characters or patterns of characters to see what looks good to you!

It looks pretty good, but you can do a bit better. Test your program by running it, and you’ll notice the number of symbols doesn’t match the length of the sentence. You’ve programmed it to display exactly 48 asterisks—no more, no less. Instead, let’s update those lines to repeat the symbol to match the length of the silly sentence. You’ll use another built-in Python function called len, which calculates the length of a string and returns a number telling you the number of characters:

print("*" * len(silly_sentence))
print(silly_sentence)
print("*" * len(silly_sentence))

That’s better! Let’s look at the code all together (see figure 3.13).

Figure 3.13. Silly Sentence Generator 3000 is a fun program that shows how programs can collect information from users, interact with them, and provide a more personal feel.

You’ve completed your program. Let’s do some final testing to see what it can do! See figure 3.14 for an example of the game’s output.

Figure 3.14. The Silly Sentence Generator 3000 makes some absurd sentences based on words you enter.

Fantastic! Feel free to update the code to add more adjectives, verbs, or nouns. You’ve learned how to get input from a computer user and interact with them by displaying a message to the screen.

Fruit Picker Extra: Minecraft Pi

In this Fruit Picker Extra, you’ll explore another unique feature of the Pi: it has its own version of Minecraft. Thanks to a collaboration between Mojang, the makers of Minecraft, and the Raspberry Pi Foundation, a free, slimmed-down version of Minecraft is available on the Raspberry Pi. Since September 2014, this version, called Minecraft Pi, is automatically installed with the Raspbian operating system.

What’s Minecraft?

Minecraft is a game that takes place in a 3D virtual world made of blocks. At the most basic level, you run around mining (digging blocks by hitting them) and crafting things (combining items in the game to make new items). You can also build things in this virtual world using different types of blocks.

Launching Minecraft Pi

Look for a Minecraft Pi icon under Menu > Games (see figure 3.15). If you got your Pi before September 2014, see the chapter 6 sidebar “Updating your Pi” to learn how to update Raspbian.

Figure 3.15. Minecraft Pi is a slimmed-down, free version of Minecraft that’s based on Minecraft Pocket Edition. It’s limited compared to the full version but still oodles of fun!

Click the Minecraft Pi icon to open the game. A Minecraft window will open (see figure 3.16). It’s a little quirky—you’ll see a black window behind the Minecraft window—but this is normal.

Figure 3.16. The Minecraft Pi main screen allows you to start a singleplayer game or join a multiplayer game. The multiplayer option lets you connect to someone else’s world, but you’ll need to be on the same network.

Click Start Game to begin to play. Next, click Create New to create a new world. After it’s done loading, you’ll find yourself in a blocky world (see figure 3.17). Each world is different, so you may see trees, water, dirt, or any number of environments.

Figure 3.17. Each Minecraft world is made of blocks but is different. You might find yourself in a forest or in a desert. The bottom of the screen shows you the items in your inventory. Use the mouse scroll wheel to select different items, or press the numbers 1–9 on your keyboard.

In Minecraft, you’re a player who can walk around using the following controls:

• W —Move forward.
• A —Move left.
• S —Move backward.
• D —Move right.
• Spacebar —Jump.
• Mouse movement —Look around or turn.
• Escape —Exit the game.

In addition to the basics, here are some other moves you may need:

• Double spacebar —Fly up in the air (double-tap the space bar and then hold it down to fly up). Press the left Shift key to move down. If you’re flying, double-tap the spacebar to fall back to the ground.
• E —Show the game inventory of blocks and items you can use (it’s limited compared to the full version of the game). Drag items you want to the small squares at the bottom of the screen. Press Escape to hide the inventory screen.
• Scroll wheel or the number 1–9 keys —Select something from one of your player inventory spots at the bottom of the screen. The item selected is in your hand for you to use.

Once you get the hang of moving around, use the mouse left click to dig or break blocks. Use the mouse right click to place a block or use the tool in your hand. When you’re ready to leave, press Escape to exit the game.

Tip

To exit Minecraft Pi, press Escape > Quit to Title, and then click the X in the corner to close the window.

Python programming interface to Minecraft Pi

Minecraft Pi has a fun inventory of materials and tools—even a sword! What’s even better is that there is a Python programming interface for Minecraft Pi. Head over to the Raspberry Pi Foundation website to learn more about how to use Python to interact with Minecraft Pi.

Explore the world, dig an underground base, or build a tree house. What will you do?

Challenges

Try these challenges to see if you can use the input function and strings to create something fun and interactive.

Knight’s Tale Creator 3000

In this challenge, try to use what you’ve learned about input (gathering text) and output (displaying text) to create a Knight’s Tale Generator. Here is a story template for you to use:

There was a brave knight, [player_name], who was sent on a quest to vanquish the [adjective] evildoer, [famous_person]. Riding on his/her trusty [animal], the brave [player_name] traveled to the faraway land of [vacation_place]. [player_name] battled valiantly against [famous_person]’s army using his [sharp_thing] until he defeated them. Emerging victorious, [player_name] exclaimed, “[exclamation]!!!” I claim the land of [vacation_place] in the name of Python.

The words in brackets are meant to be variables that you’ll create in your program; you’ll need to have the player input those words. Remember to use + to join the strings to create a unique knight’s tale, and then print the tale to the screen. Good luck!

Subliminal messages

A subliminal message is a hidden message that tries to get people to think of something you want them to think about. Often used in TV commercials, it’s a great technique to try with friends and parents to get something you want.^[3] In this challenge, try to create a message that is hidden in a large display of characters. The message should be constructed by asking for the person’s name, the name of something they want, and a pattern of letters, numbers, and symbols. In your program, you should create a message that says, “You really want to buy [player_name] a [thing]”, and hide it within a pattern of characters. Figure 3.18 shows an example.

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Use subliminal messaging at your own risk (send Ryan pizza!). If people know you’re trying to manipulate their minds, they may retaliate with subliminal messaging of their own.

Figure 3.18. The subliminal-message challenge is about hiding a secret message in a bunch of characters on the screen. Can you see the hidden message?

In this example, the hidden message is, “You really want to buy Ryan a burrito.” Be sneaky, and see if you can find a way to create and hide a subliminal message!

Summary

In this chapter, you learned how to write interactive programs that get information from a person and provide entertaining responses:

• Use the input function to collect text input from a person. Use it with a variable and an equals sign to store the information that a person types in. Here’s an example of asking the user to tell you their favorite color and saving it to a variable called favorite_color:

  favorite_color = input("What is your favorite color?")

• Add notes to your programs by starting a line with a hashtag (#) and a space:

  # A comment tells you about the code
  # They help you read the code,
  # but they are ignored by Python

• Join strings using +.
• Use parentheses when you need to join strings that are longer than a single line:

  name = input("What is your name?")
  favorite_color = input("What is your favorite color?")
  message = ("Your name is: " + name + " and your "+
             "favorite color is: " + favorite_color)

The game you created uses the same ideas to collect information from users and interact with them in the same way they see every day on websites, mobile apps, and games.