topic Re: Best way to add values to a string for UpdateCursor in Python Questions

Best way to add values to a string for UpdateCursor

JamesWilcox1970 — Tue, 06 Jul 2021 22:43:03 GMT

I am still really new to Python and ArcPy. We have a pair of feature classes, one with an index grid and the other with project locations. We are trying to create a table that shows what index grid cells each project intersects.

I have a script that performs Intersect on the two feature classes, creates a new table (indexTable) , adds the two needed fields: Label (the project number) and PageName (the index grid), and populates Label using an InsertCursor. I know that I eventually need to use an UpdateCursor with an update string to update PageName, but I am at a loss on how to properly do it, since some of the projects intersect more than one grid cell.

Currently, I have a SearchCursor to get the data from the Intersect feature class, but it breaks with "SystemError: error return without exception set".

# UpdateCursor to add all index grid cells for each label gridList = [r for r in arcpy.da.SearchCursor(intersectOut, ("Label", "PageName"))] # List from intersection arcpy.AddMessage("Grid list = {}".format(gridList)) # For debugging with arcpy.da.UpdateCursor(indexTable, ("Label", "PageName")) as uCursor: for row in uCursor: name_field = "Label" expression = u'{} = {}'.format(arcpy.AddFieldDelimiters(intersectOut, name_field), row[0]) # expression = "Label = '" + row[0] + "'" with arcpy.da.SearchCursor(intersectOut, ("Label", "PageName"), where_clause=expression) as sCursor: gridString = "" if gridString == "": gridString = gridString + sCursor[1] else: gridString = gridString + ", " + sCursor[1] arcpy.AddMessage(gridString) # Before going to the next record in the site index table write this list to the appropriate field. row[1] = gridString uCursor.updateRow(row)

I also tried using list comprehension to go through gridList, based on row[0] of the UpdateCursor, but that just returns a series of empty lists--and I was unsure where to go from there in any case.

I know there has to be a way to do this, but I am at a loss.

Re: Best way to add values to a string for UpdateCursor

DanPatterson — Tue, 06 Jul 2021 23:20:06 GMT

what is line 10 and 11 doing? you never get to 13 and 14

sample messages? or does it fail before it even gets to an addmessage?

Re: Best way to add values to a string for UpdateCursor

JamesWilcox1970 — Tue, 06 Jul 2021 23:23:00 GMT

Lines 10-14 are supposed to build the output string for the UpdateCursor...once it gets there. But it doesn't get there.

Re: Best way to add values to a string for UpdateCursor

DanPatterson — Wed, 07 Jul 2021 00:04:12 GMT

throw in some print statements since addmessage doesn't report anything?

In any event, if you are trying to figure out how to concatenate values from fixed length and potentially variable length inputs, throw them both into a list, add the lists and format accordingly

a0 = ["fixed length"] a1 = [1, 2, 3] ("{} "*len(a0+a1)).format(*(a0+a1)) 'fixed length 1 2 3 '

Re: Best way to add values to a string for UpdateCursor

BlakeTerhune — Wed, 07 Jul 2021 15:48:22 GMT

Here's a blog you should read. It's got some techniques for doing stuff like this that I have found very useful.

Turbo Charging Data Manipulation with Python Curso... - Esri Community

Basically you'll read your "look up" data into a dictionary and reference it by some id to get the value you want to write elsewhere. If I'm understanding your logic correctly, it would look something like this.

# UpdateCursor to add all index grid cells for each label gridList = {label: pageName for label, pageName in arcpy.da.SearchCursor(intersectOut, ("Label", "PageName"))} # List from intersection arcpy.AddMessage("Grid list = {}".format(gridList)) # For debugging with arcpy.da.UpdateCursor(indexTable, ("Label", "PageName")) as uCursor: for label, pageName in uCursor: gridList_pageName = gridList.get(label) arcpy.AddMessage(gridList_pageName) # Before going to the next record in the site index table # write the PageName from gridList if there is one. if gridList_pageName: uCursor.updateRow((label, gridList_pageName)) else: arcpy.AddWarning("Label {} not found in gridList; nothing updated.".format(label))

Re: Best way to add values to a string for UpdateCursor

Anonymous User — Wed, 07 Jul 2021 16:55:21 GMT

You might get a Key Error in 6 if the label is not in the gridList.

gridList_pageName = gridList.get(label, None)

Will default the gridList_pageName to None if the key is not found and will continue execution.

Re: Best way to add values to a string for UpdateCursor

BlakeTerhune — Wed, 07 Jul 2021 22:56:03 GMT

I like dict.get() because it returns None by default if the key is not found. You can specify a different value in the second argument but None is redundant.

Re: Best way to add values to a string for UpdateCursor

Anonymous User — Wed, 07 Jul 2021 23:11:36 GMT

Ah yes I forgot it does. Thanks!

Re: Best way to add values to a string for UpdateCursor

JamesWilcox1970 — Wed, 07 Jul 2021 23:32:03 GMT

Thanks for looking at it! This works great, if there were only one index grid that each project falls in. Unfortunately, using a dictionary limits me to just one index grid for each project. The issue I am having is that there are several projects that fall into multiple grids.

I will keep plugging away at it.

Thanks again!

Re: Best way to add values to a string for UpdateCursor

BlakeTerhune — Thu, 08 Jul 2021 03:27:13 GMT

@JamesWilcox1970, this is a new requirement from your initial post. Please describe all the rules in which these projects are grouped into index grids and we can help you find a solution.

Re: Best way to add values to a string for UpdateCursor

JamesWilcox1970 — Thu, 08 Jul 2021 16:19:46 GMT

I thought I had mentioned it in the original post, but I had not. 🙂 I have a feature class generated by an Intersect between Projects and Index Grids. Many of the projects intersect multiple grids, so I have multiple records in the feature class for a single project, each with a different index grid cell. I am trying to programmatically populate a field in an output table with all the index grids that a particular project intersects. The result I am aiming for is "Project: Foo, Index Grid: A1, A2, B2; Project: Bar, Index Grid: A3, B1, C5."

So I am looking for a way to loop through the projects in the output table, find each index grid that matches that project in the Intersect feature class, write it to a string, and use that string in an UpdateCursor to update the index grid field (PageName) in the output table.

I'm sorry if I was unclear in my original post. Thanks for looking at this problem.

Re: Best way to add values to a string for UpdateCursor

BlakeTerhune — Fri, 09 Jul 2021 14:04:14 GMT

I may be mixing up projects and grids but I think you get the idea. Just do the spatial join, put it into a dictionary with project as key and the value is a list of intersecting grids. Then you can do what you need with it, like write it back to your feature class as a comma separated list of values.

from collections import defaultdict try: # Perform spatial join between grid and project features. grid_project_spatialjoin_inmem = r"in_memory\grid_project_spatialjoin" arcpy.SpatialJoin_analysis( target_features=projectFeatures, join_features=gridFeatures, out_feature_class=grid_project_spatialjoin_inmem, join_operation="JOIN_ONE_TO_MANY" ) # Create dictionary keys of projects and values with list of grids project_grids = defaultdict(list) with arcpy.da.SearchCursor(grid_project_spatialjoin_inmem, ("projectPageName", "gridLabel")) as sCursor: for projectPageName, gridLabel in sCursor: project_grids[projectPageName].append(gridLabel) finally: arcpy.Delete_management("in_memory") # Write the intersecting grids for each project as comma separated list with arcpy.da.UpdateCursor(gridFeatures, ("projectPageName", "gridLabel")) as uCursor: for projectPageName, gridLabel in uCursor: grid_list = project_grids.get(projectPageName, "") uCursor.updateRow((projectPageName, ", ".join(grid_list)))

EDIT: fixed a comment for creating the dictionary

Re: Best way to add values to a string for UpdateCursor

JamesWilcox1970 — Thu, 08 Jul 2021 22:28:51 GMT

Thank you @BlakeTerhune ! It is going to take me a little bit to understand it, try it out and hammer out any bugs, but I have very high hopes now. I will update you as I go. Thanks again!

Re: Best way to add values to a string for UpdateCursor

JamesWilcox1970 — Fri, 09 Jul 2021 20:01:31 GMT

Thank you, @BlakeTerhune ! I had to do some tweaking of the code to get it to work with the data, but your idea really helped. Here's the relevant portion of the working script:

def listToString(s): # Takes input list and converts it to a string using join() # Empty string str1 = "" str1 = ", ".join(s) return str1 try: # Perform spatial join between grid and project features. grid_project_spatialjoin_inmem = r"in_memory\grid_project_spatialjoin" arcpy.SpatialJoin_analysis( target_features=culFC, join_features=quadFC, out_feature_class=grid_project_spatialjoin_inmem, join_operation="JOIN_ONE_TO_MANY" ) arcpy.AddMessage("Join created.") # Create dictionary of grid keys and value with list of projects project_grids = defaultdict(list) with arcpy.da.SearchCursor(grid_project_spatialjoin_inmem, ("Label", "PageName")) as sCursor: for Label, PageName in sCursor: project_grids[Label].append(PageName) finally: arcpy.Delete_management("in_memory") arcpy.AddMessage("Dictionary created.") for key, value in project_grids.items(): project_grids[key] = str(listToString(value)) # Changes the value to a string, but class is still list? grid_list = project_grids.items() # New dictionary from project_grids grid_list = list(grid_list) # Convert it to a list # for item in grid_list: # Checking list contents # arcpy.AddMessage("{}: {}.".format(item[0], item[1])) with arcpy.da.InsertCursor(indexTable,("Label", "PageName")) as iCursor: for row in grid_list: iCursor.insertRow(row) arcpy.AddMessage("Processing complete.")

Thank you again for all the help!

Jim