GIS Programming Module 4: Debugging and Error Handling

This week's programming topic was debugging, or finding and fixing errors in code, and error handling, which means writing code that assumes certain kinds of errors may occur and "traps" them, allowing the code to complete anyway.

For the assignment, we were given three pre-written Python scripts containing errors, two of which we had to fix so that the code would run correctly, and the last of which was an exercise in error handling. All of the scripts this week use ArcGIS geoprocessing functions, so this was also my first official exposure to how Python and ArcGIS interact.

The corrected first script takes a shapefile and prints a list of the field names in the attribute table. It works something like this: import ArcGIS modules > assign workspace (map document) > assign feature class > make list of fields > for each item in fields list, print field name

Here are the results:

The second script is a bit more complex and was harder to fix, with more errors and more complicated ones. (I also ran into some added complications on this script and the third one because I'm working from my local computer rather than the UWF GIS remote desktop and thus have to change the file paths in the scripts, which introduces opportunities for new errors!) I'm not sure about everything this script is doing, because we haven't learned much about the geoprocessing functions yet, but in the end, it prints a list of the layers within a specified data frame:

I was nervous about the error handling part of the assignment, but it turned out to be very easy--a generic try-except statement rather than trying to deal with specific kinds of errors separately. The third script has two parts, the first of which is supposed to turn on the visibility and labels for a layer within an ArcMap document, but is written so that it raises an exception. I enclosed most of the code for this section of the script in the try-except statement, which means that the script now prints "Error:" and the error message for this first part, before continuing the run the second part, which prints the name, spatial reference, and scale for the specified data frame:

Another issue I had this week is that I was using IDLE as my Python editor instead of the recommended PythonWin, and I have two versions of it on my computer, a standalone copy and the one that installs along with ArcGIS. The standalone version doesn't recognize arcpy, the module containing the GIS-specific functions, but it is the one my scripts open in by default, so I kept getting error messages about arcpy that weren't part of the assignment. So frustrating! But at least I figured it out, and now know that if I want to use IDLE, I have to open the right version. Or maybe I should just stick to PythonWin.