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There are many modules in Python that provide very powerful features that we can use in our own programs. Some of these can send email or fetch web pages. Others allow us to perform complex mathematical calculations. In this chapter we will introduce a module that allows us to create a data object called a turtle that can be used to draw pictures.
Turtle graphics, as it is known, is based on a very simple metaphor. Imagine that you have a turtle that understands English. You can tell your turtle to do simple commands such as go forward and turn right. As the turtle moves around, if its tail is down touching the ground, it will draw a line (leave a trail behind) as it moves. If you tell your turtle to lift up its tail it can still move around but will not leave a trail. As you will see, you can make some pretty amazing drawings with this simple capability.
Note
The turtles are fun, but the real purpose of the chapter is to teach ourselves a little more Python and to develop our theme of computational thinking, or thinking like a computer scientist. Most of the Python covered here will be explored in more depth later.
Let’s try a couple of lines of Python code to create a new turtle and start drawing a simple figure like a rectangle. We will refer to our first turtle using the variable name alex, but remember that you can choose any name you wish as long as you follow the naming rules from the previous chapter.
The program as shown will only draw the first two sides of the rectangle. After line 4 you will have a straight line going from the center of the drawing canvas towards the right. After line 6, you will have a canvas with a turtle and a half drawn rectangle. Press the run button to try it and see.
Here are a couple of things you’ll need to understand about this program.
The first line tells Python to load a module named turtle. That module brings us two new types that we can use: the Turtle type, and the Screen type. The dot notation turtle.Turtle means “The Turtle type that is defined within the turtle module”. (Remember that Python is case sensitive, so the module name, turtle, with a lowercase t, is different from the type Turtle because of the uppercase T.)
We then create and open what the turtle module calls a screen (we would prefer to call it a window, or in the case of this web version of Python simply a canvas), which we assign to variable wn. Every window contains a canvas, which is the area inside the window on which we can draw.
In line 3 we create a turtle. The variable alex is made to refer to this turtle. These first three lines set us up so that we are ready to do some drawing.
In lines 4-6, we instruct the object alex to move and to turn. We do this by invoking or activating alex’s methods — these are the instructions that all turtles know how to respond to.
Complete the rectangle ...
Modify the program by adding the commands necessary to have alex complete the rectangle.
Mixed up programs
turtle-2-2: The following program uses a turtle to draw a capital L as shown in the picture to the left of this text, but the lines are mixed up. The program should do all necessary set-up: import the turtle module, get the window to draw on, and create the turtle. Remember that the turtle starts off facing east when it is created. The turtle should turn to face south and draw a line that is 150 pixels long and then turn to face east and draw a line that is 75 pixels long. We have added a compass to the picture to indicate the directions north, south, west, and east.
Blank lines separate the code into "blocks" - keep each block together, but put them in the correct order.
turtle-2-3: The following program uses a turtle to draw a checkmark as shown to the left, but the lines are mixed up. The program should do all necessary set-up: import the turtle module, get the window to draw on, and create the turtle. The turtle should turn to face southeast, draw a line that is 75 pixels long, then turn to face northeast, and draw a line that is 150 pixels long. We have added a compass to the picture to indicate the directions north, south, west, and east. Northeast is between north and east. Southeast is between south and east.
turtle-2-4: The following program uses a turtle to draw a single line to the west as shown to the left, but the program lines are mixed up. The program should do all necessary set-up: import the turtle module, get the window to draw on, and create the turtle. The turtle should then turn to face west and draw a line that is 75 pixels long.
An object can have various methods — things it can do — and it can also have attributes — (sometimes called properties). For example, each turtle has a color attribute. The method invocation alex.color(“red”) will make alex red and the line that it draws will be red too.
The color of the turtle, the width of its pen(tail), the position of the turtle within the window, which way it is facing, and so on are all part of its current state. Similarly, the window object has a background color which is part of its state.
Quite a number of methods exist that allow us to modify the turtle and window objects. In the example below, we show just show a couple and have only commented those lines that are different from the previous example. Note also that we have decided to call our turtle object tess.
The last line plays a very important role. The wn variable refers to the window shown above. When we invoke its exitonclick method, the program pauses execution and waits for the user to click the mouse somewhere in the window. When this click event occurs, the response is to close the turtle window and exit (stop execution of) the Python program.
Each time we run this program, a new drawing window pops up, and will remain on the screen until we click on it.
Extend this program ...
- Modify this program so that before it creates the window, it prompts the user to enter the desired background color. It should store the user’s responses in a variable, and modify the color of the window according to the user’s wishes. (Hint: you can find a list of permitted color names at http://www.w3schools.com/colors/colors_names.asp. It includes some quite unusual ones, like “PeachPuff” and “HotPink”.)
- Do similar changes to allow the user, at runtime, to set tess’ color.
- Do the same for the width of tess’ pen. Hint: your dialog with the user will return a string, but tess’
pensizemethod expects its argument to be anint. That means you need to convert the string to an int before you pass it topensize.
turtle-2-8: Can you create code to produce this image?
turtle-2-9: The following program uses a turtle to draw a capital L in white on a blue background as shown to the left,
but the lines are mixed up. The program should do all necessary set-up and create the turtle and set the pen size to 10. The turtle should then turn to face south, draw a line that is 150 pixels long, turn to face east, and draw a line that is 75 pixels long. Finally, set the window to close when the user clicks in it.
turtle-2-10: The following program uses a turtle to draw a capital T in white on a green background as shown to the left, but the lines are mixed up. The program should do all necessary set-up, create the turtle, and set the pen size to 10. After that the turtle should turn to face north, draw a line that is 150 pixels long, turn to face west, and draw a line that is 50 pixels long. Next, the turtle should turn 180 degrees and draw a line that is 100 pixels long. Finally, set the window to close when the user clicks in it.
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