Saving rasters in a Python for loop fails only on last iteration using ArcPy with ArcMap

Question

I'm a newcomer to Python scripting.

I'm using a for loop to save rasters to a specified folder. The loop executes perfectly, but the last iteration of the loop invariably produces an empty raster with no spatial reference. So, iteration 1 will be an empty raster until I run iteration 2, at which point the product of iteration 1 mysteriously transforms into what I want, & so on w/ iterations 2 & 3, etc. In keeping w/ this pattern, the last raster created by the loop remains empty and unprojected.

Code is as follows ("costsurf" & "distDirString" are defined in an earlier part of the script):

for Num in Nums:
 focal = arcpy.sa.SetNull("Corr","Corr","VALUE <> " + str(Num))
 costDist = arcpy.sa.CostDistance(focal,costsurf)
 withhole = arcpy.sa.SetNull(~ arcpy.sa.IsNull(focal),costDist)
 withhole.save(os.path.join(distDirString,"dist_%s.img" %Num))

It's supposed to pull out a chunk of the raster "Corr" with cell values Num by setting all else to NoData, run the Cost Distance tool on it, then poke a hole in the result where the source cells are. I have no idea how to think about what's failing here, since the loop does exactly what I want it to, but only if it's not the last iteration to be run.

It seems to behave itself if I get rid of the ".img" and save them as GRIDs but only sometimes, and I'd really rather stick to .img if at all possible.

I'm using ArcMap 10.0.

Answer 1

I seem to recall working with ArcPy that some optimizations are at work, such as not writing data to disk when it may be altered later.

Try adding withhole = None at the end of the loop, or after the loop, telling arcpy that it now must write the raster.