How do you add a long list of rasters together in Python?

Question

If you have produced a long list of rasters (lets say 50) and want to add them together, is there any shorter way to write in python rather than typing all the inputs, like so:

outRas = inRas1 + inRas2.........+ inRas50

Answer 1

You can make a list of the raster names you wish to add together, then step through the list, adding each one to an output raster.

import arcpy
from arcpy import env

# Set the current workspace
# 
env.workspace = "C:/Data/DEMS"

#Check out ArcGIS Spatial Analyst extension license
arcpy.CheckOutExtension("Spatial")


# Get a list of ESRI GRIDs from the workspace and print
#

rasterList = arcpy.ListRasters("*", "GRID")

#Step through the list of raster names
for rasname in rasterList:
    #Cast rasname as raster before adding
    listras = Raster(rasname)
    #Add each raster to an output raster.
    #The previous step of casting as a Raster will invoke
    #the Spatial Analyst Addition function.
    outras += listras

#Save output Raster
outras.save("C:/temp/outras")