Using ArcPy to get layer symbology?

Question

Since ArcGIS 10 comes with the ArcPy package, I'm wondering it is possible to use ArcPy functions to get Symbology (i.e color, width...) of a layer?

Answer 1

For ArcGIS 10.0, if you can make use of a read-only approach, the following code sample reveals the field (and joined table) that forms the basis for the symbology of a layer. It exports a temporary msd(a zip containing xml files) and loads the specific attributes into an object. These classes might be extended to gain access to additional layer attributes.

import zipfile
from arcpy import mapping
import os
from xml.dom.minidom import parse


class LayerExtras(object):
    """ An object to hold attributes loaded from xml inside the msd."""

    name = ""
    symbologyFieldName = ""


class MxdExtras(dict):
    """ Exposes extra MXD details by raiding an exported msd

        Treat this object as a dictionary with layer name as the key and a custom object
        with desired attributes as the value.
        You must have write access to MXD directory (creates temporary msd file).
        Only layers in the first dataframe are accessed.

    """    

    LYR_NAME_NODE = "Name"
    LYR_SYMBOL_NODE = "Symbolizer"
    LYR_FIELD_NODE = "Field"
    MSD_SUFFIX = "_MxdExtrasTemp.msd"
    MXD_SUFFIX = ".mxd"
    EXCLUDED_FILE_NAMES = ["DocumentInfo.xml", "layers/layers.xml"]
    mxdPath = ""

    def __init__(self, mxdPath):

        self.loadMxdPath(mxdPath)


    def loadMxdPath(self, mxdPath):
        """ Load mxd from file path """

        self.mxdPath = mxdPath.lower()
        mxd = mapping.MapDocument(self.mxdPath)

        msdPath = self.mxdPath.replace(self.MXD_SUFFIX, self.MSD_SUFFIX) 

        # Delete temporary msd if it exists
        if os.path.exists(msdPath):
            os.remove(msdPath)

        mapping.ConvertToMSD(mxd,msdPath)

        zz = zipfile.ZipFile(msdPath)

        for fileName in (fileName for fileName in zz.namelist() if not fileName in self.EXCLUDED_FILE_NAMES):
            dom = parse(zz.open(fileName))
            name, lyr = self.loadMsdLayerDom(dom)
            self[name] = lyr
        del zz
        os.remove(msdPath)

    def loadMsdLayerDom(self, dom):
        """ Load dom created from xml file inside the msd. """

        lyr = LayerExtras()  

        # Layer name
        lyr.name = dom.getElementsByTagName(self.LYR_NAME_NODE)[0].childNodes[0].nodeValue

        # Symbology field name
        symbologyElement = dom.getElementsByTagName(self.LYR_SYMBOL_NODE)[0]
        lyr.symbologyFieldName = symbologyElement.getElementsByTagName(self.LYR_FIELD_NODE)[0].childNodes[0].nodeValue

        return lyr.name, lyr


############
# Test

if __name__ == "__main__":

    mxdPath = r"c:\temp\AmphibianSpeciesRichnessAverageOf30mCells.mxd"

    mxde = MxdExtras(mxdPath)

    for lyr in mxde.itervalues():
        print "Layer Name: ", lyr.name 
        print "Layer Symbology Field Name: ", lyr.symbologyFieldName
        print 

Example output of test:

Layer Name:  Amphibian Species Richness Average of 30m Cells
Layer Symbology Field Name:  biodiversity.AmphAve

Answer 2

ArcPy looks to let you change the symbology, but only with existing .lyr files, and not specify the symbols directly in your code based on my reading of the module.

Answer 3

In ArcGIS 10.1 and newer there is direct access to the symbology via the symbology property of the layer object.

For ArcGIS 10.0 the mentioned workarounds did work for me.