Debugging ArcGIS Python scripts

Question

I am new to Python and coding generally, and I cannot figure out how to set up a debugger that will work with Arcpy. I am trying to use PyCharm, but it always gets caught up with the fact that it can't actually open ArcMap. It gives me this error when I run in the IDE:

RuntimeError: Object: CreateObject cannot open map document.

I run the scripts from a Toolbox and they don't run correctly. All I know how to do is comment out some lines and narrow the problem down by re-running. There's got to be a better way. Right?

• Blerg: Does 'import arcpy' work for you in PyCharm?
• gene: You don't need PyCharm. ArcPy is a Python module like others and there are many solutions for debugging scripts (look at Debugging Python Like a Boss for example). pdb is a standard module (in all Python distributions)
• agarfield: It does. All PyCharm is to me right now is a text editor that (usually) autocompletes Arcpy.
• Michael Stimson: There's nothing wrong with wanting autocomplete (intelisense) as it can be a real timesaver especially as python is case sensitive. I use PyWin (on Win7) and that will autocomplete after you've imported arcpy in the interactive window. As @Nir said, can you import arcpy in your IDE/Editor? If you can't open a map document then there's possibly something wrong with your syntax... note that opening a document from python is not the same as launching ArcMap (you wont see the document open) - are you trying to write geoprocessing scripts or python plugins? (yes, there's a huge difference)
• Blerg: Following up on Michael, When you type "import arcpy" and execute, do you get an error?

Answer 1

If your scripts are failing at lines where you're using the arcpy package, you can use try & except statements. In the try statement you should put your arcpy methods, and in the except you can write out arcpy.GetMessages() to a log file.

arcpy.getMessages() writes out the messages from the last geoprocessing event attempted. It may help you identify where your script is going wrong.

• gene: It you really want to debug a script, it is not the Python proper way, use a debugger module as pdb

Answer 2

paste your python script into the python window in arcmap and execute it from there. the error massage will tell you what line the error occurs on.

Answer 3

For large scripts I use Eclipse. Eclipse is a very powerful, respected and free IDE that can be used for a number of languages. Eclipse can also play nicely with source control such as Subversion etc, which is handy for larger projects where a few developers are working together. For Python development you need the PyDev plugin. Setting up Eclipse for development with ArcPy is fairly straightforward. There are many tutorials out there and a quick search yields this one.

Answer 4

I tend to put major bits of py in try statements. The "AddMessage()" and "AddError()" and "except Exception as e:" are all rather handy at reporting different things.

 desc = arcpy.Describe(raster)
 SR = desc.spatialReference
 try:
 arcpy.AddMessage(" - Band Count: %d" % desc.bandCount)
 txtRasterFile.write (desc.bandCount+",")
 except:
 arcpy.AddMessage(" - Does Not Support Bands")
 txtRasterFile.write ("Unsupported,")
 try:
 arcpy.AddMessage(" - Compression Type: %s" % desc.compressionType)
 txtRasterFile.write (desc.compressionType+",")
 except:
 arcpy.AddMessage(" - Does Not Support Compression")
 txtRasterFile.write ("Unsupported,")
 try:
 arcpy.AddMessage(" - Raster Format: "+desc.format)
 txtRasterFile.write (desc.format+",")
 except:
 arcpy.AddError(" - Raster Format: Error") #Error Message
 txtRasterFile.write ("Error,")
 try:
 arcpy.someprocess()
 except Exception as e:
 arcpy.AddError(" - Processes Faled: "+e.message)