Applying consistent symbology across all fields of layer using ArcPy?

Question

I have a shapefile of a country with 50 fields. Each field contains values (-3 to 3) for a particular month. I would like to create a for loop that iterates through the fields, updates the map, uses a common symbology and exports to a PNG file.

The symbology I would like to use is one color per number. -3 = dark orange, -2 = light orange, -1 = light yellow, 0 = grey, 1 = light green, 2 = dark green, 3 = very dark green. I've assigned these colors to the layer in the .mxd, and created a .lyr.

So far, I have this:

import arcpy
mxd = arcpy.mapping.MapDocument(r'path/to/mxd')
shp = r'path/to/country_shapefile'
outdir = r'path/to/out/directory'
symbology = r'path/to/symbology.lyr'

df = arcpy.mapping.ListDataFrames(mxd)[0]
fields = [field.name for field in arcpy.ListFields(shp)]
fields = fields(17:len(fields) # I only need to display the data in fields 17 onwards
layer = arcpy.mapping.ListLayers(mxd, 'country')[0]
symbol = layer.symbology

for field in fields:
    symbol.valueField = field
    symbol.classValues = [-3,-2,-1,0,1,2,3] # The range of values in the symbology
    arcpy.ApplySymbologyFromLayer_management(layer, symbology)
    arcpy.RefreshActiveView()
    arcpy.RefreshTOC()
    arcpy.mapping.ExportToPNG(mxd, outdir+'\\'+field+".png")

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When I try this, I get a map displaying the values in every field, but not with the correct symbology.

Answer 1

As I commented, I don't think you can use arcpy.ApplySymbologyFromLayer_management() and symbol.valueField to change the field but keep the same symbols.

A possible work-around would be to create a new field to store your values in for symbolising. This field could be temporary if you can't leave it in the shapefile, or alternatively if you can't modify the shapefile you could copy the entire dataset to an in_memory layer for processing purposes.

The idea is to loop through your different fields and copy the values to your symbology field. Your symbology properties will then stay the same, but the actual values on the feature will change each time the script loops.

Create your layer file first, with the symbology you want to use (set to a field called temp_field or whatever you want to call your temporary field

Sample Data:

Layer file symbology properties:

import arcpy, os
mxd = arcpy.mapping.MapDocument("CURRENT")
outdir = r"C:\GIS\SE\SymOutput"
symLayer = r'C:\GIS\SE\LayerToSymboliseFrom.lyr'
myLayer = arcpy.mapping.ListLayers(mxd)[0]
arcpy.AddField_management(myLayer,'temp_field', "SHORT")
fields = [field.name for field in arcpy.ListFields(myLayer) if field.name <> 'temp_field']
fields = fields[2:len(fields)]
for field in fields:
    arcpy.CalculateField_management(myLayer, "temp_field", "!{0}!".format(field), "PYTHON_9.3")
    arcpy.ApplySymbologyFromLayer_management(myLayer, symLayer)
    arcpy.RefreshActiveView()
    arcpy.mapping.ExportToPNG(mxd, os.path.join(outdir, "{0}.png".format(field)))
arcpy.DeleteField_management(myLayer, 'temp_field')

Screenshots showing the output:

Field 1:

Field 2:

Field 3:

Answer 2

here are a few remarks to move further (see my comments on your script):

for field in fields: # this returns some fields objects, but you need to work with fields names (this can be managed here or later)
    arcpy.ApplySymbologyFromLayer_management(layer, symLayer) #symLayer is a lyr file with your symbology 
    layer.symbology.valueField = field.name #this must be changed AFTER you import the symbology, and I use the name of the field
    arcpy.RefreshActiveView()
    arcpy.RefreshTOC()
    arcpy.mapping.ExportToPNG(mxd, outdir+'\\'+field.name+".png")#here again, use fields' name