[Python] ArcPy symbology ignores ManualInterval and defaults to StandardDeviation — how to...

I'm using ArcPy with ArcGIS Pro 3.3.1 to apply a graduated color symbology to a raster or feature layer, using custom class breaks and labels. According to the ArcGIS GraduatedColoursRenderer Documentation, I should be able to set:

sym.renderer.classificationMethod = "ManualInterval"

But even after doing so, ArcGIS Pro still displays the layer as classified using StandardDeviation, both in the legend and in the layer properties dialog. The breaks labels appear correctly, but the underlying breaks and classification method are ignored or reset.

Here’s the simplified version of my function:

def apply_symbology(self, layer, field_name, breaks, labels):

sym = layer.symbology
sym.updateRenderer("GraduatedColorsRenderer")
sym.renderer.classificationField = field_name
sym.renderer.classificationMethod = "ManualInterval"
sym.renderer.breakCount = len(breaks) - 1
layer.symbology = sym # required to initialize classBreaks

# Manually override breaks
sym = layer.symbology
for i in range(len(breaks) - 1):
brk = sym.renderer.classBreaks
brk.upperBound = breaks[i + 1]
brk.description = labels

layer.symbology = sym


Yet the result in the map still shows Standard Deviation as the classification method, even though breaks and labels are applied. What I’ve tried:

Confirmed that breaks and labels are applied programmatically.

Used layer.symbology = sym after modification to force update.

Tried setting the classification method both before and after breaks.

Using other methods like "EqualInterval" works — but "ManualInterval" is ignored.

Manually changing the method in ArcGIS Pro after running the script does work — so this seems like an ArcPy-specific bug or undocumented behavior.


My Questions:

Is "ManualInterval" no longer supported or broken in recent ArcGIS Pro versions?

Is there an alternative way to fully enforce manual classification using ArcPy?

How can I prevent ArcGIS from defaulting back to "StandardDeviation"?


I'm using:

ArcGIS Pro: 3.3.1

Python: 3.9.x (ArcGIS Pro default)

Renderer: GraduatedColorsRenderer on a feature layer


Thanks!

Heres some details about the values in my dataset and the sort of input the apply_symbology function is recieving.

Maximum value in field: 2.243088768755757 Rounded value to nearest 0.5: 2.5 Breaks: [0.0, 0.417, 0.833, 1.25, 1.667, 2.083, 2.5] Labels: ['0-0.42%', '0.42-0.83%', '0.83-1.25%', '1.25-1.67%', '1.67-2.08%', '2.08-2.50%']

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