Swampy Tutorial

Check your installation

To see if Swampy is installed correctly, launch the Python interpreter and try:

>>> from swampy.TurtleWorld import *
>>> world = TurtleWorld()
>>> world.mainloop()

If that works, you are all set. Try out some of the other examples below.

Otherwise, follow these instructions to install Swampy.


This example demonstrates how a beginning programmer, who only knows how to translate mathematical expressions into Python, can use Amoeba world to practice Python syntax and have some fun with it.
  1. To start AmoebaWorld, move into the directory that contains the Swampy code and type
    python AmoebaWorld.py
    Press the 'Run' button. You should see something like this:

    The pink blob is meant to be an Amoeba on a microscope slide. The coordinates of the slide run from -10 to 10 in each direction. As the Amoeba moves, it leaves a trail of slime in its path. Press 'Clear' to erase the slime.

  2. The fields x(t) and y(t) are mathematical expressions that control the position of the Amoeba as a function of time. By default, x(t) = y(t) = t, so the Amoeba traces a straight line with x = y.

    Type the expression 10 * math.cos(t) in the x(t) field, and 10 * math.sin(t) in the y(t) field. Press 'Run' again. The Amoeba should make a circle.

  3. As the program runs, debugging information appears in the window where you ran the program. Make sure you can see this window before you proceed.

    Make a deliberate syntax error—for example, delete the * in one of the fields—and press 'Run'. An error message appears in the window where you ran the program. Swampy displays unvarnished Python error messages so that students learn to read them as early as possible.

  4. Press 'Quit' or close the window to exit.


This example demonstrates the path from user to programmer. Student use the TurtleWorld GUI to learn the turtle API, then run short programs in a text field, then write longer programs in separate files.
  1. To start TurtleWorld, move into the directory that contains the Swampy code and type
    python TurtleWorld.py

  2. Press 'Make Turtle'. A new turtle should appear along with a Turtle Control Panel. Use the buttons to move the turtle around the screen:
    fd      move forward
    bk      make back
    lt	turn left
    rt	turn right
    pu	pen up (don't draw lines)
    pd	pen down
    Use the color menu to change the color of the turtle.

  3. Now press 'Run code' to execute these statements:
    bob = Turtle(world)
    The new turtle, bob, doesn't have a control panel. To make him move, you have to write code. For example, modify the program to read
    bob = Turtle(world)
    for i in range(100):

    In Python, spaces and tabs are important, so the last two lines have to be indented by the same amount. You can cut and paste from this web page into the text field.

    Before you press 'Run code', see if you can figure out what the turtle will do. Then try it.

  4. The text field is useful for experimenting with short programs, but it is better to store long programs in a separate file. An example is provided in turtle_code.py.

    Press 'Clear' to clear the screen, then 'Run file' to execute turtle_code.py. You should see something like this:

    This is a classic example of a recursive function that demonstrates the self-similar structure of many natural patterns (in this case, cauliflower).

  5. Press 'Quit' or close the window to exit.
The TurtleWorld exercises follow the order of presentation in How to Think Like a Computer Scientist, which is a Free Textbook that comes with Swampy.


This example demonstrates the two primary uses of Lumpy: diagramming small test programs in order to develop a model of execution, and analyzing the structure of a larger program.
  1. The file lumpy_example1.py uses Lumpy to generate a UML object diagram. It contains the following code:
    # create a Lumpy object
    import Lumpy
    lumpy = Lumpy.Lumpy()
    # run the test code
    x = [1, 2, 3]
    y = x
    z = list(x)
    # draw the current state

    To run it, move into the directory that contains the Swampy code and type:

    python lumpy_example1.py
    You should see something like this:

    The green boxes represent the lists. The blue box represents a stack frame, which is labelled __main__ because the variables were declared in the top-level scope, which is named __main__.

    Lumpy uses a simple layout algorithm, but it provides drag-and-drop capability so the user can fine-tune the appearance of the diagrams. Notice that moving a frame also moves the contents of the frame, and moving a variable also moves its value.

  2. To generate a Postscript version of this diagram, press 'Print to file'. By default, the diagram is stored in lumpy.ps. You can print it or view it with a Postscript viewer.

  3. Use a text editor to open lumpy_test.py, and add the following code after the line z = list(x) (you can cut-and-paste from this web page):
    d = dict(a=1, b=2, c=3)
    t = d.items()

    Run lumpy_test.py again. You should see a diagram showing that d is a dictionary (purple) that maps the letters a, b and c to the numbers 1, 2, 3; and t is is list of tuples (dark green), where each tuple is an item from the dictionary.

  4. To see a more substantial example, type
    python lumpy_example2.py

    TurtleWorld should appear, then another window with an object diagram showing bob, world, and the objects and values they contain.

    When you close the object diagram, a class diagram appears. The blue arrows indicate inheritance ("is-a" relationships); the orange arrows represent reference ("has-a" relationships).

  5. Normally Lumpy avoids diagramming itself by declaring the Lumpy class "opaque", meaning that the contents of a Lumpy object are not shown. In lumpy_example3.py, Lumpy is made transparent. To run it, type
    python lumpy_example3.py

    This example demonstrates the limitations of Lumpy's simple graph layout algorithm.


This example demonstrates the behavior of a generalized Turing machine called Langton's Ant. It is used to introduce cellular automata, and to demonstrate the complex behavior that can come from simple rules.
  1. To start TurmiteWorld, move into the directory that contains the Swampy code and type
    python TurmiteWorld.py
    Press 'Make Turmite'. A red triangle should appear, representing Langton's Ant, a four-state turing machine that lives on a 2-dimensional, 2-state grid. During each time step, Langton's Ant follows this program:

    • If the current cell is yellow, turn right. Otherwise turn left.

    • Toggle the current cell (black becomes yellow and vice versa).

    • Move forward one cell.

    Press 'Step' to see the turmite execute one time step.

  2. Press 'Run' to let the turmite run. The turmite follows a complex and unpredictable path for more than 10 000 steps. After that, the turmite repeats a sequence of 104 steps, each time shifting by one cell toward the upper left corner. The resulting structure is called "the highway". When the turmite gets to the highway, press 'Stop'.

  3. The code in the text field creates three turmites facing in different directions. Press 'Clear' and then 'Run code' to execute this program. In this case, one of the turmites finds the highway, but a second turmite chases him down, and their interaction put the first turmite in reverse, consuming the highway it just built. These two fall into a repeating pattern, building and destroying the highway. Meanwhile, the third turmite makes his escape.

  4. While the program is running, press 'Make Turmite' and see what effect the fourth turmite has.

  5. Press 'Quit' or close the window to exit.


Sync is a simulator intended for use with The Little Book of Semaphores, a Free Textbook that comes with Swampy. The following examples demonstrate solutions to three of the problems in the book.
  1. Sync.py contains the Sync simulator; mutex.py, readwrite.py and coke.py contain scripts Sync can execute. To load mutex.py into the simulator, move into the directory that contains the Swampy code and type
    python Sync.py mutex.py
    You should see something like this:

    The first three lines contain initialization code that is executed once. The next four lines contain synchronization code that uses a semaphore to ensure exclusive access to the "critical section", which contains an increment operation (which is usually not thread-safe).

    The red circle represents a thread. When a thread is in a Run area, it is about to execute the corresponding line of code. When it is in a Queue area, it is waiting for another thread to signal the semaphore.

  2. Press 'Step' four times to see the thread execute the program once. When it finishes, it loops back to the top.

  3. Press 'Create thread' to make a second thread (synchonization is more interesting with more than one thread). Click on Thread B to advance it one step. You should see something like this:

    The value of the semaphore is 0, which means that it is locked; since Thread B is in the critical section, additional threads cannot enter.

  4. Click on Thread A to make it try to enter the critical section. It should move into the Queue area, and decrement the semaphore.

  5. Click on Thread B three times. The third time, it leaves the critical section, which increments the semaphore and allows Thread A to proceed.

  6. Press 'Random Run'. The simulator chooses threads at random and allows them to run. If the semaphore is working, there should never be more than one thread in the critical section.

  7. Edit the line that says mutex.wait() so that it says mutex.signal(). This is no longer a correct implementation of mutual exclusion, since threads don't wait to enter the critical section. Press 'Random Run' again, and the chances are that you will soon see more than one thread in the critical section.

  8. Close the window to exit.

  9. Sync comes with other examples in the directory sync_code. For example, readwrite.py is a solution to the reader-writer problem and coke.py is a solution to the producer-consumer problem (here in the form of a coke machine).

    To run these examples, type

    python Sync.py sync_code/readwrite.py


    python Sync.py sync_code/coke.py

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