Use list element to build new path

Question

I'm currently trying to build two lists containing folder names, in order to use their elements for geoprocessing tools later on. However I struggle with creating the lists. I'm using ArcGIS 10.3 (Python 2.7).

What I need:

C:\test contains multiple subfolders that I would like to put in the first list, i.e. C:\test\test1, C:\test\test2, etc. The first set of subfolders contains another set, i.e. C:\test\test1\xy1, C:\test\test1\xy2 etc. What I need is a list with the names, not the paths, to each of the first level subfolders as in firstlevelList = [test1, test2, test3] and secondlevelList = [xy1, xy2, xy3]. I then want to use the arcpy.MosaicToNewRaster_management function to process data in each of the secondlevelList separetely (sort of according to this question).

What I've tried:

import arcpy, os

workspace = r"C:\test"
arcpy.env.workspace = workspace
arcpy.env.scratchworkspace = workspace
arcpy.env.overwriteOutput = True


firstlevelList = []
firstlevelList.append(os.listdir(workspace))
## up to here, everything works very well, the list contains all the available subfolders in my workspace.

secondlevelList = []
for element in firstlevelList:
    newPath = """{0}\{1}""".format(workspace, element)
    secondlevelList.append(os.listdir(newPath))

## This creates the following error:
##Runtime error
##Traceback (most recent call last):
##  File "<string>", line 8, in <module>
##WindowsError: [Error 3] Das System kann den angegebenen Pfad nicht finden: "C:\\Users\\sutter\\Desktop\\GIS_temp\\test\\['test1', 'test2', 'test3']/*.*"

I understand that apparently I cannot use a list element to create a new path? I then also tried to following, which worked for one element of the fistlevelList() but not for all of them... :

secondlevelList = []
i = 0
for element in firstlevelList:
     newPath = """{0}\{1}""".format(workspace, element[i])
     secondlevelList.append(os.listdir(newPath))
     i += 1

Am I using the os.listdir function the wrong way? My knowledge about the os.path / os.walk functions is rather limited but I guess they could also help? Would I be better off using the arcpy.ListFiles() function?

The aim of the script: Finally I would like to create a mosaic for all rasters in the secondlevelList separately. Example: folder C:\test\test1\xy1 contains 3 rasters. They need to be mosaiced. Then the following folder needs to be processed C:\test\test1\xy2 etc. I wrote this code for doing this:

for tile in firstlevelList:
    for run in secondlevelList:
        zeitschrittRaster = arcpy.ListRasters("*", "TIF")
        arcpy.MosaicToNewRaster_management(zeitschrittRaster, Sammelordner, """{0}.tif""".format(run), "", "32_BIT_FLOAT", 10, 1, "MAXIMUM", "FIRST")

Answer 1

There are dedicated list functions in arcpy and I would use them rather than the python os.listdir function:

import arcpy, os

workspace = r"C:\test"
arcpy.env.workspace = workspace
arcpy.env.scratchworkspace = workspace
arcpy.env.overwriteOutput = True

firstlevelList = arcpy.ListWorkspaces(workspace_type = "Folder")

for element in firstlevelList:
    arcpy.env.workspace = element
    secondlevelList = arcpy.ListFiles()
    for file in secondlevelList:
        # do something

file represents the file itself even though if you print it you will only get its name. You can input it to a tool without having to recreate the full path to the file. Nevertheless if you want to do this for some reason, add this line under the last for block:

path_to_file = os.path.join(ws, file)

Depending on the type of the files (rasters?) and what you want to do with them, another list function might be more approriate:

• arcpy.ListRasters()
• arcpy.ListDatasets()
• arcpy.ListFeatureClasses()
• ...

See also the help page for arcpy.ListWorkspaces() and arcpy.ListFiles().

Answer 2

Maybe this can help you: Using Python to execute merge of rasters

I used os.path.join and os.walk (os path)