Using the Debugger

console-based Python debugger

If you are stuck trying to find the error in your python program, using a debugger like pudb can help. A debugger let's you step through your program, one statement at a time, examining variables as you go. It is often more efficient than adding lots of print statements.

Here's how you start pudb for a simple python program called blastoff.py:

$ pudb blastoff.py

That should change the terminal window to like this:

The above shows the program source code, as well as other frames for current variables (empty, since we are not running the program yet), the stack, and breakpoints.

setting a breakpoint

We often want to run to a certain point in the program, then stop and examine variables, or proceed one step at a time. This is done with breakpoints. Using the arrow keys to move the cursor to line 10, setting a breakpoint with the 'b' key, then running the program ('r'), results in this:

Things to note:

• the program ran (asked for input...not shown), then stopped at line 10
• the variables frame now shows start = 5 (the user entered 5 on line 8)
• the breakpoint frame shows a list of current breakpoints

execute line by line

At this point we may want to keep running the program one line at a time. Hitting the n key runs the next line of code (iterations of the for loop). Here's what things look like after hitting the n key 7 times:

Notice we have executed the for loop a few times: i currently equals 2. If you keep hitting n, you will eventually finish the for loop and hit the print("blastoff") line. You will also see that i never gets all the way down to the value 1 (the "bug" in this example).

summary

Using breakpoints and "next", we can step though a program one line at a time, seeing what each variable is, as we go.

$ pudb myprogram.py
- set a breakpoint (use arrow keys to highlight line, then hit b key)
- run code to breakpoint (r key)
- run next line of code (n key)