topic Nesting a While Loop Within a For Loop using arcpy.da.UpdateCursor() in Python Questions

Nesting a While Loop Within a For Loop using arcpy.da.UpdateCursor()

daveostrander — Wed, 30 Nov 2022 15:06:38 GMT

Hi Community,

I am attempting to use the arcpy.Geometry() objects .union() method to update records in a feature class that were appended but now need to be merged with an existing feature where appropriate (governed by a SQL expression). I am using the arcpy.da.UpdateCursor() to perform the updates. I am attempting to do something like the following, but I am getting an "AttributeError: 'Array' object has no attribute 'union'" error.

arcpy.MakeFeatureLayer_management(zoning, 'zoning_lyr') arcpy.management.SelectLayerByAttribute('zoning_lyr', "NEW_SELECTION", "Zoning = 'A'") selectCount = int(arcpy.GetCount_management('zoning_lyr')[0]) if selectCount >= 2: zoningGeom = arcpy.CopyFeatures_management('zoning_lyr',arcpy.Geometry()) edit.startEditing(False, False) edit.startOperation() N = 1 with arcpy.da.UpdateCursor(zoning, "Shape@", "Zoning = 'A'") as cursor: for row in cursor: if N == selectCount:break while N < selectCount: row = row[0].union(zoningGeom[N]) # Fails here cursor.updateRow([row]) N += 1 del row, cursor edit.stopOperation() edit.stopEditing(True) arcpy.Delete_management('zoning_lyr')

The code below works without error, except it doesn't run through the whole list of zoningGeom features before moving to the next zoning record in the for loop. Ideally I would have it run through the records in the zoningGeom geometry object, unioning each one to the zoning layer before running out of features to union and exiting both loops.

N = 1 with arcpy.da.UpdateCursor(zoning, "Shape@", "Zoning = 'A'") as cursor: for row in cursor: if N == selectCount:break row = row[0].union(zoningGeom[N]) cursor.updateRow([row]) N += 1

I think I am just missing something fundamental here. Hopefully, someone can swoop in and point me in the right direction or, better yet, offer a solution.

Thank you in advance,

Re: Nesting a While Loop Within a For Loop using arcpy.da.UpdateCursor()

DanPatterson — Wed, 30 Nov 2022 01:26:40 GMT

edit and format your code would be helpful

Code formatting ... the Community Version - Esri Community

Re: Nesting a While Loop Within a For Loop using arcpy.da.UpdateCursor()

daveostrander — Wed, 30 Nov 2022 02:22:10 GMT

It doesn’t appear to be possible on mobile. I will update/correct tomorrow. Thank you for the tip.

Re: Nesting a While Loop Within a For Loop using arcpy.da.UpdateCursor()

JohannesLindner — Wed, 30 Nov 2022 06:38:14 GMT

Your first script fails because you forgot the index in the failing line. So you're trying to use the non-existing union method of an array (row) instead of the geometry (row[0]).

But the script would fail there anyway, because arcpy.management.CopyFeatures() does not what you think it does. You seem to want to extract the geometries, but this tool copies a feature class from one location to another.

You got the gist of what you need to do:

Extract the geometries
Iterate over the rows with an UpdateCursor
1. union the current geometry with all extracted geometries
2. write it into the feature class
3. optionally: you probably should delete every row after the first one, so you don't get overlapping features.

zones = "TestPolygons" # layer name or fc path zone_name_field = "TextField" # name of the field that determines the "sameness" only_keep_first = True # only keep the first row of each zone, delete the following rows zone_names = ["A"] # Names you want to union # You can also do it automatically for every zone name: #zone_names = {r[0] for r in arcpy.da.SearchCursor(zones, [zone_name_field])} for zone_name in zone_names: sql = f"{zone_name_field} = '{zone_name}'" # extract the geometries into a list zone_geometries = [r[0] for r in arcpy.da.SearchCursor(zones, ["SHAPE@"], sql)] first_row = True # iterate over the fc with arcpy.da.UpdateCursor(zones, ["SHAPE@"], sql) as cursor: for r in cursor: if first_row: # union the current geometry with every extracted geometry geo = r[0] for other_geo in zone_geometries: geo = geo.union(other_geo) cursor.updateRow([geo]) # delete the following rows if only_keep_first == True first_row = not only_keep_first else: cursor.deleteRow()

Before:

With only_keep_first == True:

With only_keep_first == False:

Re: Nesting a While Loop Within a For Loop using arcpy.da.UpdateCursor()

daveostrander — Wed, 30 Nov 2022 17:55:34 GMT

Not only the solution I was after but served on a silver platter!!!

Thank you @JohannesLindner