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I'm a newcomer to Python scripting and I'm having what's seemingly a very simple problem but one that's stumped me enough to send me posting to these forums for the first time.

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 ="Corr","Corr","VALUE <> " + str(Num))
 costDist =,costsurf)
 withhole =,costDist),"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, which it surely must be. I'm using ArcGIS 10.0 btw. Thanks for your help!

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up vote 5 down vote accepted

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

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Brilliant, thank you sir, you are a lifesaver. I had a feeling there'd be a simple solution, but I don't think I would've thought to simply assign something unrelated to the variable at the end. I suppose it kind of makes sense, though I don't think I've seen it do this w/ saving not in a loop. Good to know. – user14175 Jan 12 '13 at 19:25

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