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I am attempting to loop through two folders, one of input files (watershed polygons) and erase features (area boundaries) using arcpy.Erase_analysis

Here is my current code:

in_cover = "G:/Files/Watersheds"

erase_cover = "G:/Files/Areas"

out_cover = "G:/Output/Erase_output/"



arcpy.env.workspace = in_cover 
    for fc in [os.path.join(in_cover, x) for x in arcpy.ListFeatureClasses()]:
         print fc
          arcpy.env.workspace = erase_cover
          for fc2 in [os.path.join(erase_cover, y) for y in arcpy.ListFeatureClasses()]:
               print fc2
                 arcpy.Erase_analysis(in_features = fc, erase_features = fc2,
                                         out_feature_class = out_cover + "UNP_" + fc)

Running this, I get this error:

ExecuteError: Failed to execute. Parameters are not valid. ERROR 000725: Output Feature Class: Dataset G:\Output\Erase_output\XXX1.shp already exists.

It seems like the second loop is returning all files in the folder instead of one. For example, if i remove the erase function and just print, here's what happens:

arcpy.env.workspace = in_cover 
        for fc in [os.path.join(in_cover, x) for x in arcpy.ListFeatureClasses()]:
            print fc
            arcpy.env.workspace = erase_cover
            for fc2 in [os.path.join(erase_cover, y) for y in arcpy.ListFeatureClasses()]:
                print fc2

This returns:

G:/Files/Watersheds\WS1.shp
G:/Files/Areas\Area1.shp
G:/Files/Areas\Area2.shp
G:/Files/Areas\Area3.shp
G:/Files/Areas\Area4.shp
G:/Files/Areas\Area5.shp
G:/Files/Watersheds\WS2.shp
G:/Files/Areas\Area1.shp
G:/Files/Areas\Area2.shp
G:/Files/Areas\Area3.shp
G:/Files/Areas\Area4.shp
G:/Files/Areas\Area5.shp
G:/Files/Watersheds\WS3.shp
G:/Files/Areas\Area1.shp
G:/Files/Areas\Area2.shp
G:/Files/Areas\Area3.shp
G:/Files/Areas\Area4.shp
G:/Files/Areas\Area5.shp

...And so on.

How can I fix this?

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1
  • I thought that was the behavior you wanted. Your description reads as if you want to iterate over the watersheds and erase all the areas from each watershed. What result do you want?
    – Tom
    Nov 21, 2019 at 16:28

1 Answer 1

Reset to default
2

ListFeatureClasses doesn't return the full path to the feature class. It only returns the name. Then, within the outer loop, you change workspaces. So, in your call of Erase, you're passing G:/Files/Areas/[fc] instead of then intended G:/Files/Watersheds/[fc].

To resolve this, where you iterate over the feature classes in "in_cover", use a list comprehension to create the full path:

for fc in [os.path.join(in_cover, x) for x in arcpy.ListFeatureClasses()]:
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3
  • This helps with the issue of incorrectly iterating over the input names instead of paths. Thank you. However, now the second for loop is not working because it is spitting back the first iteration of the loop everytime. For example, if the first loop goes to watershed1, then watershed2..etc - the second for loop only returns area1 every time or essentially the entire list of shapefile areas. Any idea how to fix this?
    – Fully Aquatic
    Nov 20, 2019 at 19:17
  • @Ecostrider, can you update your question to show your updated code? Thanks.
    – Tom
    Nov 20, 2019 at 21:41
  • I updated the question
    – Fully Aquatic
    Nov 21, 2019 at 14:04

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