Wow... that was quick. Thank you, @DanPatterson, for pointing me to Esri's Cartographic Information Model.
The section Additional Resources and Sample Scripts links to the GitHub repository and a sample project with a script to create a dashed effect:
# Author: Esri
# Date: March 2020
# Version: ArcGISPro 2.5
# Purpose: This script creates or modifies the dash effects for a line symbol.
# Notes: - The script is intended to work from a script tool provided with
# a sample project using "CURRENT". To see the changes happen be
# sure to activate the appropriate map or layout.
# - In addition to this sample script demonstating how the CIM
# provides access to capabilities not in the managed Python API,
# is also demonstrates a method for creating CIM objects that are
# necessary to set desired properties.
p = arcpy.mp.ArcGISProject('current')
m = p.listMaps('Symbology')[0]
l = m.listLayers('States_SingleSymbol')[0]
l_cim = l.getDefinition('V2') #Get the Layer's CIM definition
#Symbol Level 1 (Solid Stroke)
symLvl1 = l_cim.renderer.symbol.symbol.symbolLayers[0]
#If a dashed effect does not exist, create one
if len(symLvl1.effects) == 0:
newDash = arcpy.cim.CreateCIMObjectFromClassName('CIMGeometricEffectDashes', 'V2')
newDash.dashTemplate = [5, 5]
symLvl1.effects = [newDash]
#If a dashed effects does exist, modify it
else:
dash = symLvl1.effects[0]
dash.dashTemplate = [10, 10]
l.setDefinition(l_cim) #Set the Layer's CIM definition
This serves as a good starting point. Do you know of an easy way to expose the JSON structure of existing symbol styles? Or how to gain insight into the hierarchy?
