Think about the previous example where you ran some map algebra on an elevation raster. If you wanted to change the value of your cutoff elevation to 2500 instead of 3500, you had to open the script itself and change the value of the cutoffElevation variable in the code.
This third example is a little different. Instead of hard-coding the values needed for the tool (in other words, literally including the values in the script) we’ll use some user input variables, or parameters. This allows people to try different values in the script without altering the code itself. Just like in ModelBuilder, parameters make your script available to a wider audience.
The simple example below just runs the Buffer tool, but it allows the user to enter the path of the input and output datasets as well as the distance of the buffer. The user-supplied parameters make their way into the script with the arcpy.GetParameterAsText() method.
Examine the script below carefully, but don't try to run it yet. You'll do that in the next part of the lesson.
# This script runs the Buffer tool. The user supplies the input # and output paths, and the buffer distance. import arcpy arcpy.env.overwriteOutput = True try: # Get the input parameters for the Buffer tool inPath = arcpy.GetParameterAsText(0) outPath = arcpy.GetParameterAsText(1) bufferDistance = arcpy.GetParameterAsText(2) # Run the Buffer tool arcpy.Buffer_analysis(inPath, outPath, bufferDistance) # Report a success message arcpy.AddMessage("All done!") except: # Report an error messages arcpy.AddError("Could not complete the buffer") # Report any error messages that the Buffer tool might have generated arcpy.AddMessage(arcpy.GetMessages())
Again, examine the above code line by line and figure out as much as you can about what the code does. If necessary, print the code and write notes next to each line. Here are some of the main points to understand:
- GetParameterAsText() is a function in the arcpy module. Notice that it takes a zero-based integer (0, 1, 2, 3, etc.) as an argument. If you’re going to go ahead and make a tool out of this script, as we are going to do in the next page of this lesson, then it’s important you define the parameters in the same order you want them to appear in the tool’s dialog.
- When we called the Buffer tool in this script, we supplied only three parameters. By not supplying any more, we accepted the default values for the rest of the tool’s parameter (Side Type, End Type, etc.).
- The try and except blocks of code are a way that you can prevent your script from crashing if there is an error. Your script attempts to run all of the code in the try block. If the script cannot continue for some reason, it jumps down and runs the code in the except block. Inserting try/except blocks like this is a good practice to do once you think you've gotten all the errors out of your script, or when you want to make sure your code will run a certain line at the end no matter what happens.
When you are first writing and debugging your script, sometimes it's more useful to leave out try/except and let the code crash, because the (red) error messages reported in the Interactive Window sometimes give you better clues on how to diagnose the problem in your code. Suppose you put a print statement in your except block saying "There was an error. Please try again." For the end user of your script, this is nicer than seeing a nasty (red) error message; however, as a programmer debugging the script, you want to see the (red) error message to get any insight you can about what went wrong.
Projects that you submit in this course require error handling using try/except in order to receive full credit.
- The arcpy.AddMessage() and arcpy.AddError() methods are ways of adding additional messages to the user of the tool. Whenever you run a tool, the geoprocessor prints messages, which you have probably seen before (for example, “Executed (Buffer) successfully. End time: Sat Oct 03 07:37:31 2009”). You have the power to add more messages through these methods. The messages have differing levels of severity, hence different methods for AddMessage and AddError. Sometimes people choose to view only the errors instead of all the messages, or they do a quick visual scan of the messages for errors.
When you use arcpy.GetMessages(), you get all the messages generated by the tool itself. These will tell you things such as whether the user entered invalid parameters. Notice in this script the somewhat complex syntax you have to use to first get the messages, then add them: arcpy.AddMessage(arcpy.GetMessages()). If this line of code is confusing to understand, remember that the order of functions works like math operations: you start by working inside the parentheses first to get the messages, then you add them.
The AddError and AddMessage methods are only used when making script tools (which you'll learn about in the very next section). When you are just running a script in PythonWin (not making a script tool), you can still get the messages using a print statement with GetMessages(), like this: print arcpy.GetMessages().
Read the section of Chapter 5 that talks about working with tool messages (5.10) for another perspective on handling tool output.