Error handling advice needed for Python arcpy script

Question

I could use some advice on how to handle errors in my Python script. From what I have been able to gather from reading all of the Python error handling posts on this site, is that you cannot simply bypass an error within a for loop with a try except continue statement. Instead, you have to handle each error directly. This is where I am having problems tying it all together. I have attached the error message that occured in the middle of a for loop. Additionally, I have attached my script which moves through the following workflow:

place a polygon around the raster
place a point on the mean center of the polygon
use the point to identify a specific county associated with its corresponding raster
clip the raster based on the selected county polygon

How do I incorporate information from the error message into a try except continue statement, so that the script can move to the next raster in the list rather than stopping in the middle of processing?

# Import arcpy module
import arcpy
from arcpy import env
from arcpy.sa import *
arcpy.CheckOutExtension("3D")

# Set Over write
arcpy.env.overwriteOutput = 1

# Set the workspace
env.workspace = r"Z:\temp.gdb"
outworkspace = r"Z:\location2\temp2.gdb"

# Local variables:
counties = r"Z:\temp.gdb\boundaries\Counties"
counties_lyr = arcpy.MakeFeatureLayer_management(counties,"counties_lyr")

# Get the list of rasters to process
raster_list = arcpy.ListRasters("*_clp")
print raster_list

for raster in raster_list:
    # Define name and location for output raster
    name = outworkspace + "\\" + str(raster)

    # Process: Raster Domain
    arcpy.RasterDomain_3d(raster, "in_memory/temp", "POLYGON")

    # Process: Central Feature
    arcpy.MeanCenter_stats("in_memory/temp", "in_memory/temp1")

    # Process: Select Layer By Location
    arcpy.SelectLayerByLocation_management(counties_lyr, "intersect", "in_memory/temp1", "", "NEW_SELECTION")

    # Clip Raster
    arcpy.Clip_management(raster, "#", name,counties_lyr, "#", "ClippingGeometry")

    # Delete in_memory
    arcpy.Delete_management("in_memory")

    print "processing " + raster + " complete..."

print "All processing is now finished"

enter image description here

Mark Amery · Accepted Answer · 2012-10-12 18:38:20Z

Is this what you need? It will print out the traceback if anything goes wrong with any individual raster in the for loop, and then move on to the next one.

import traceback

# Import arcpy module
import arcpy
from arcpy import env
from arcpy.sa import *
arcpy.CheckOutExtension("3D")

# Set Over write
arcpy.env.overwriteOutput = 1

# Set the workspace
env.workspace = r"Z:\temp.gdb"
outworkspace = r"Z:\location2\temp2.gdb"

# Local variables:
counties = r"Z:\temp.gdb\boundaries\Counties"
counties_lyr = arcpy.MakeFeatureLayer_management(counties,"counties_lyr")

# Get the list of rasters to process
raster_list = arcpy.ListRasters("*_clp")
print raster_list

for raster in raster_list:
    try:
        # Define name and location for output raster
        name = outworkspace + "\\" + str(raster)

        # Process: Raster Domain
        arcpy.RasterDomain_3d(raster, "in_memory/temp", "POLYGON")

        # Process: Central Feature
        arcpy.MeanCenter_stats("in_memory/temp", "in_memory/temp1")

        # Process: Select Layer By Location
        arcpy.SelectLayerByLocation_management(counties_lyr, "intersect", "in_memory/temp1", "", "NEW_SELECTION")

        # Clip Raster
        arcpy.Clip_management(raster, "#", name,counties_lyr, "#", "ClippingGeometry")

        # Delete in_memory
        arcpy.Delete_management("in_memory")

        print "processing " + raster + " complete..."

    except:
        print "Something went wrong handling " + str(raster) + ". Here's a traceback:"
        traceback.print_exc()
        continue

print "All processing is now finished"

This is a lazy programmer's implementation, wrapping the entire contents of the for loop in a try... except... block that will catch exceptions of any type anywhere in the block. Depending upon your needs and tastes, you may find it more helpful (or simply more elegant) to catch only the specific error that you're getting in practice, as dm03514 suggested - but I figure that for a little script like this whose output is going to be viewed by a human, it doesn't matter which you do.

What probably IS important to you, as you specify in the question, is that you see the traceback when an error occurs. That's where traceback.print_exc() comes in. :)

dm03514 · Accepted Answer · 2012-10-12 18:20:22Z

1

you can handle errors in a for loop. Wrapping your code in try except block will allow you to handle the error in case paramters are not valid.

try:

# Process: Raster Domain
    arcpy.RasterDomain_3d(raster, "in_memory/temp", "POLYGON")

except ExcecuteError: 
  continue

This will take place inside your loop its basically saying if the exception happens issue a continue which means go on to the next loop iteration.

You can place the try except block anywhere, As Mark Amery suggested you could wrap the whole for loop body in try except too.

answered Oct 12, 2012 at 18:20

dm03514

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Comments

jazz · Accepted Answer · 2012-10-12 18:23:59Z

1

I do not know arcpy. But maybe handling the exception in line 969 with a:

try:
    your stuff

except arcpy.ExecuteError, error:
    print error

could help.

answered Oct 12, 2012 at 18:23

jazz

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Comments

Smith · Accepted Answer · 2012-10-12 18:43:01Z

Not quite clear on what you're saying about try/except/continue in for loops. The following is loosely based on the code you provide.

def failing_func(raster):
    raise ValueError("raster <%s> function has failed" % (raster))

def working_func(raster):
    print("raster <%s> function has succeeded" % (raster))
    return True

raster_list = [0, 1, 2, 3, 4, 5]

for raster in raster_list:
    print("Processing <%s>" % (raster))

    try:
        v1 = working_func("raster_1")
        v2 = failing_func("raster_2")
        v3 = working_func("raster_3")
        v4 = failing_func("raster_4")
        v5 = working_func("raster_5")
    except ValueError, e:
        print("Exception was raised: %s when processing item %s" % (str(e), raster))
        continue

Which outputs:

Processing <0>
raster <raster_1> function has succeeded
Exception was raised: raster <raster_2> function has failed when processing item 0
Processing <1>
raster <raster_1> function has succeeded
Exception was raised: raster <raster_2> function has failed when processing item 1
Processing <2>
raster <raster_1> function has succeeded
Exception was raised: raster <raster_2> function has failed when processing item 2
Processing <3>
raster <raster_1> function has succeeded
Exception was raised: raster <raster_2> function has failed when processing item 3
Processing <4>
raster <raster_1> function has succeeded
Exception was raised: raster <raster_2> function has failed when processing item 4
Processing <5>
raster <raster_1> function has succeeded
Exception was raised: raster <raster_2> function has failed when processing item 5

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