Zigzag

Let’s use the programming concepts you’ve learned so far to create a small animation program. This program will create a back-and-forth, zigzag pattern until the user stops it by pressing by pressing CTRL-C. When you run this program, the output will look something like this:

    ********
   ********
  ********
 ********
********
 ********
  ********
   ********
    ********

Enter in the following code in the editor:

import time, sys
indent = 0 # How many spaces to indent.
indentIncreasing = True # Whether the indentation is increasing or not.

try:
    while True: # The main program loop.
        print(' ' * indent, end='')
        print('********')
        time.sleep(0.1) # Pause for 1/10 of a second.

        if indentIncreasing:
            # Increase the number of spaces:
            indent = indent + 1
            if indent == 20:
                # Change direction:
                indentIncreasing = False

        else:
            # Decrease the number of spaces:
            indent = indent - 1
            if indent == 0:
                # Change direction:
                indentIncreasing = True
except KeyboardInterrupt:
    sys.exit()

Let’s look at this code line by line, starting at the top.

import time, sys
indent = 0 # How many spaces to indent.
indentIncreasing = True # Whether the indentation is increasing or not.

First, we’ll import the time and sys modules. Our program uses two variables: the indent variable keeps track of how many spaces of indentation are before the band of eight asterisks and indentIncreasing contains a Boolean value to determine if the amount of indentation is increasing or decreasing.

try:
    while True: # The main program loop.
        print(' ' * indent, end='')
        print('********')
        time.sleep(0.1) # Pause for 1/10 of a second.

Next, we place the rest of the program inside a try statement. When the user presses CTRL-C while a Python program is running, Python raises the KeyboardInterrupt exception. If there is no try-except statement to catch this exception, the program crashes with an ugly error message. However, for our program, we want it to cleanly handle the KeyboardInterrupt exception by calling sys.exit(). (The code for this is in the except statement at the end of the program.)

The while True: infinite loop will repeat the instructions in our program forever. This involves using ' ' * indent to print the correct amount of spaces of indentation. We don’t want to automatically print a newline after these spaces, so we also pass end='' to the first print() call. A second print() call prints the band of asterisks. The time.sleep() function hasn’t been covered yet, but suffice it to say that it introduces a one-tenth-second pause in our program at this point.

if indentIncreasing:
            # Increase the number of spaces:
            indent = indent + 1
            if indent == 20:
                indentIncreasing = False # Change direction.

Next, we want to adjust the amount of indentation for the next time we print asterisks. If indentIncreasing is True, then we want to add one to indent. But once indent reaches 20, we want the indentation to decrease.

        else:
            # Decrease the number of spaces:
            indent = indent - 1
            if indent == 0:
                indentIncreasing = True # Change direction.

Meanwhile, if indentIncreasing was False, we want to subtract one from indent. Once indent reaches 0, we want the indentation to increase once again. Either way, the program execution will jump back to the start of the main program loop to print the asterisks again.

except KeyboardInterrupt:
    sys.exit()

If the user presses CTRL-C at any point that the program execution is in the try block, the KeyboardInterrrupt exception is raised and handled by this except statement. The program execution moves inside the except block, which runs sys.exit() and quits the program. This way, even though the main program loop is an infinite loop, the user has a way to shut down the program.