topic Re: Error setting field for UniqueValueRenderer in Python Questions

Error setting field for UniqueValueRenderer

TorbjørnDalløkken2 — Tue, 09 May 2023 13:20:34 GMT

Hi.

I'm trying to change the symbology of one of my layers in a python script. I've found the given layer

# Get layer "Vegteig" from active map aprx = arcpy.mp.ArcGISProject("CURRENT") m = aprx.activeMap fl_vegteig = m.listLayers("Vegteiger")[0]

Then I'm trying to define a new UniqueValueRenderer for the layer:

symb = fl_vegteig.symbology symb.updateRenderer("UniqueValueRenderer") symb.renderer.fields = ['TILKNYTTET']

When I try to run this, I get this error:

The attribute 'fields' is not supported on this instance of UniqueValueRenderer

Does anyone know what might be wrong? I'm using almost the same code in another script (same toolbox), which works perfectly.

Re: Error setting field for UniqueValueRenderer

Anonymous User — Fri, 12 May 2023 13:02:10 GMT

There is one post on SO that mentions that the layers datasource was broken and he was getting this error. I'm sure you worked that out already so maybe it would be helpful to report this to esri? No too long ago there was another question on here about changing values and the method to do so in the docs were not working so maybe there is something really wrong with the UniqueValueRenderer class.

Re: Error setting field for UniqueValueRenderer

DuncanHornby — Wed, 06 Sep 2023 00:02:55 GMT

Hi,

Did you find a solution to this? I'm developing some code in Arcpro 3.1.3 and I too get the same vague error. I've tried running it in the python command line window as well as notebook. I've also saved the project, re-opened it with the layer in it and tried running the code below. The layer exists, so its not a broken layer,there is sensible data in it and SubNetID field exists. If I go to the Symbology panel in ArcPro and set it to unique value, subnetid, then it all uniquely colours so I know the dataset is OK. Dare I say it that ESRI have somehow broken this with an update?

My code is simply:

aprx = arcpy.mp.ArcGISProject("Current") aprxMap = aprx.listMaps("Map")[0] layer = aprxMap.listLayers('Sub1000_3')[0] # This layer was initially loaded into the map as a single symbol aSymbology = layer.symbology aSymbology.updateRenderer('UniqueValueRenderer') aSymbology.renderer.fields = ["SubNetID"] p = arcpy.mp.ArcGISProject("CURRENT") cr = p.listColorRamps("Basic Random")[0] aSymbology.colorRamp = cr layer.symbology = aSymbology

It fails on line 6 with

NameError: The attribute 'fields' is not supported on this instance of UniqueValueRenderer.

Trying to think out of the box, you could argue the renderer has not been given any values to render but it seems to be a chicken and egg situation, how can I give it values when the first step is to say which field you want to render on, but its that line it fails on!

I've tagged a few of python gurus into this to see if they can identify the issue: @DanPatterson_Retired , @curtvprice , @XanderBakker , @jcarlson , @JoshuaBixby

Re: Error setting field for UniqueValueRenderer

curtvprice — Fri, 08 Sep 2023 20:56:30 GMT

The first code sample in the help does it exactly the way you're doing it, so I'm kind of mystified too.

Re: Error setting field for UniqueValueRenderer

DuncanHornby — Mon, 11 Sep 2023 18:47:12 GMT

I've been exploring this problem and it appears to be an issue with the number of unique values. I need to symbolise 13,000 values and this could easily be more. The GUI handles it but when I set it to be an UniqueValueRenderer in arcpy it blows up with a misleading error message. The property sampleSize does not appear when accessing the renderer properties (that sees like a cosmetic bug to me) as I can set it, but it has no affect.

I wondered about going down the CIM route but because the object model is not clear and there appears to be zero examples anywhere on the internet of setting the symbology of a layer to be unique values using CIM in arcpy it is impossible for me to test my code as EVERYTIME I try something ArcPro crashes throwing me out into Windows...

It would be good if esri released some code samples of using CIM to create an UniqueValueRenderer in arcpy. All I have seen on the internet are c# solutions.

Quite frustrating.

Re: Error setting field for UniqueValueRenderer

TorbjørnDalløkken2 — Mon, 15 Jan 2024 13:23:06 GMT

@DuncanHornby @curtvprice Did you guys find any solutions? I can't figure this out. I have a couple of python scripts which after some updates sets a uniquevaluerenderer as described in the first post. In one of the scripts (using the same layer) it works, and in the other script it fails. Unfortunatly this error seems to be happening in ArcGIS Pro 3.2 as well.

Re: Error setting field for UniqueValueRenderer

DuncanHornby — Mon, 15 Jan 2024 14:07:08 GMT

No, I came to the conclusion its some sort of bug based upon number of unique values, so my code just traps the error and fails gracefully.

Re: Error setting field for UniqueValueRenderer

makitahideki — Wed, 24 Jul 2024 21:09:57 GMT

Recently, I had to change the layer symbology using CIM. I didn't use UniqueValues, but I hope this helps.

Note: The value of "val['renderer']" was obtained from the .lyrx file (to get the symbology settings, I had to change the file extension to '.json').

for item, val in dict_feat.items(): layer = mapFrame.map.addDataFromPath(val['shape']) layer.name = item cim_lyr = layer.getDefinition('V2') cim_lyr.renderer = val['renderer'] layer.setDefinition(cim_lyr)

Re: Error setting field for UniqueValueRenderer

mattkramer — Mon, 21 Apr 2025 22:13:08 GMT

Adding on to note that I have experienced this kind of bug when attempting to set a field on a layer with 0 features. Seems to error out on both extremes (too many records, or too few).

Re: Error setting field for UniqueValueRenderer

Samy_BoumaNgock — Tue, 27 May 2025 13:52:46 GMT

Just a note that I have a working script as part of a broader implementation:

Feature class is of type Point (e.g. Centroid of countries)
Field name must be the internal field name (e.g. Country_Flag), not alias name (e.g. Country Flag). It is also case sensitive
Country_Flag field contains 238 unique values
Step 1: set default renderer with default 'Shape Marker' symbol, based on Country_Flag field values
Step 2: modify each symbol with their corresponding 'Picture Marker', based on images of country flags stored in a folder on disk. Each image's filename corresponds to exactly a value name in the Country_Flag field

def set_defaultuniquevaluesrenderer(featurelayer=None, fieldname=None): """Set a default unique values renderer for the specified field in the feature layer.""" if featurelayer is None: arcpy.AddError("Layer not found. Symbology not updated.") sym = featurelayer.symbology sym.updateRenderer('UniqueValueRenderer') sym.renderer.fields = [fieldname] featurelayer.symbology = sym arcpy.AddMessage(f"Symbology updated to UniqueValueRenderer on field: {fieldname}") def set_updatesymbology(featurelayer=None, fieldname=None, imagefolder=None): """Update only the point symbols as picture markers with corresponding images.""" if featurelayer is None: arcpy.AddError("Layer not found. Symbology not updated.") # Get the CIM (Cartographic Information Model) definition of the layer cim = featurelayer.getDefinition('V3') renderer = cim.renderer # Iterate through each class in the renderer's first group for classDef in renderer.groups[0].classes: # Get the value for the symbol (usually the flag image filename) try: value = classDef.values[0].fieldValues[0] except (AttributeError, IndexError, TypeError): arcpy.AddWarning(f"Skipping class with no valid value for symbol: {classDef.name}") continue # Build the full path to the image file imagePath = os.path.join(imagefolder, value) if arcpy.Exists(imagePath): # Read and encode the image as base64 with open(imagePath, "rb") as imgFile: encoded = base64.b64encode(imgFile.read()).decode("utf-8") # Define a CIMPictureMarker using the encoded image pictureMarker = { "type": "CIMPictureMarker", "enable": True, "size": 10, "url": f"data&colon;image/png;base64,{encoded}", } # Create a CIMPointSymbol with the picture marker pointSymbol = { "type": "CIMPointSymbol", "symbolLayers": [pictureMarker] } # Create a CIMSymbolReference for the class symbol symbolRef = { "type": "CIMSymbolReference", "symbol": pointSymbol } # Assign the new symbol to the class definition classDef.symbol = symbolRef # Apply the updated CIM definition back to the feature layer featurelayer.setDefinition(cim) arcpy.AddMessage("Point symbols updated with corresponding flag images.")

(view in My Videos)

Re: Error setting field for UniqueValueRenderer

AlanTonkinFMC — Tue, 21 Oct 2025 12:37:53 GMT

Just came across this and have found it super useful. I have successfully used the script to load images as symbols for around 400 unique class values. What I have searched for and not found, is how to set the Quality attribute to "Picture" rather than the "Draft" that it defaults to.

I added in another line between where the Size and the URL are set as per below:

"quality": 'Picture',

I also tried 'Picture Quality' and neither option had any affect.

There are too many to go and set this manually for 400 unique values. Any ideas?