Loop through input and erase features arcpy.Erase_analysis()

Question

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?

• Tom: 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?

Answer 1

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()]:

• Fully Aquatic: 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?
• Tom: @Ecostrider, can you update your question to show your updated code? Thanks.
• Fully Aquatic: I updated the question