CS 0008:  Spring 2015 Lab 1

YOUR NAME:

YOUR PARTNER'S NAME:

To get credit on this lab, attend recitation, work with your partner to answer the questions, and give "reasonable" (if not all correct) answers.  Each of you should hand in a hardcopy of your own  solution, due at the beginning of next week's recitation.

Note:  The Debugger in IDLE is more "full service", but
http://pythontutor.com looks better.  Feel free to experiment with
both of them.

************************************
***** Part 1: Opening Debugger
************************************

Open up IDLE, and click on "Debug" on the toolbar at the top. From the dropdown, select "Debugger" to open a new window; this is the Debugger window.

If you return to Python shell window and click on the "Debug" toolbar dropdown, you should now see a checkmark next to "Debugger." Additionally, the Python shell window should now have written "[DEBUG ON]".

************************************
***** Part 2: Using the debugger
************************************

Open up a new python file window by using the CTRL+N shortcut, or by going to FILE > New Window. For the purpose of this lab, we will work with a simple python program dealing with arithmetic and simple variables. Using the debugger is very helpful, because it shows you the current value of any and every variable you have.

Copy and paste the following code into your new python window, making sure to remove any tabs or spaces from the beginning of the lines:

a = 5
b = 2
c = a + b
c = c + a * 2
c = c + 1

Now that you have the debugger open, run the code that you just pasted in by hitting F5, or by going to Run > Run Module. Any changes you have made to a program will need to be saved before the program can be run, and so if you haven't saved already, IDLE will ask you to save your new file.

Once you've run the file, the Python shell window will jump into focus, and the debugger will pop up. Keep clicking on "Step" - Python will run one line of code at a time, showing the values of the variables as your code is executed.

Specifically, notice that, as your program executes, the variables a, b, and c will appear under the section named "Locals." The label "Locals" is a common shortening of "local variables" which is just a collection of every variable that exists at the current location in your code. If you also look at the middle select window, you can see that, as you click the step button, the line number will change:

> '__main__'.<module>(), line 3: c = a + b

to

> '__main__'.<module>(), line 4: c = c + a * 2

This will show you where you currently are within your program! Run through your program to make sure that you can follow how each variable is being computed and that each line of the Python code makes sense.

NOTE: When you step to a line of code, that line of code has not yet been executed. For instance, when you are currently on line 3, you should notice that "Locals" does not contain the variable 'c' yet; only when you hit "step" again will the debugger execute that code and create the new variable. In the beginning, it might feel like you are looking at everything one step behind!

************************************
***** Part 3: Finding Errors
************************************

Now that you know how to work the debugger, let's see how it responds to errors.

Go to the python file you were previously working on and replace the code with the following:

a = 1
b = 0
c = a / b
d = c + "2"

Make sure the debugger is open and once again run the program. Then go step by step through the program.

As you will notice, a highlighted error message will pop up complaining about a "ZeroDivisionError" and the program will not finish running.

Errors like a "ZeroDivisionError" will cause a program to crash, so you won't be able to look at what the rest of the program does until you fix it!

Replace the code with the following and debug it again:

a = 1
b = 0
c = a * b
d = c + "2"

Once again the program will crash due to an error, this time because strings (a word) can't be added to an integer (a whole number).

Using the debugger in this way will let you find and correct errors as they appear.