[GIS] ListFields (arcpy) not reading from layer

arcgis-10.1arcpy

I have a very simple python script that opens an mxd from a folder and then loops through each layer, and finds a ObjectID field and changes the alias name. 

#setup
import arcpy
from arcpy import env

#Go into folder where mxd's are
env.workspace = r"D:\Temp\Test_Mxd"
#for each mxd
for mxd in arcpy.ListFiles("*.mxd"):
    print "Working on map document: " + mxd
    mapdoc = arcpy.mapping.MapDocument("D:\\Temp\\Test_Mxd\\" + mxd)
    #   for each dataframe
    df = arcpy.mapping.ListDataFrames(mapdoc,"Layers")[0]
    ##layers = arcpy.mapping.ListLayers(mapdoc,"*",df):
    layers = arcpy.mapping.ListLayers(mapdoc,"*",df)

    #   for each layer
    for layer in layers:
        print "Working on layer: " + layer.name
        **fieldList = arcpy.ListFields(layer)**
        for field in fieldList:
            if field.name == "OBJECTID":
        # Set the new alias name
                field.aliasName == "IGNORE::OBJECTID"
    mapdoc.save()
del mxd

my problem is on the line fieldList = arcpy.ListFields(layer), I get a error in the interpreter (pythonwin) which says that the layer does not exist.
Has anyone got any idea on where I am going wrong?

Best Answer

The documentation here says that ListFields takes a String parameter. So I would try with the datasource path, rather than a layer object itself. I.e. something like:

if layer.supports("DATASOURCE"):
    fieldList = arcpy.ListFields(layer.dataSource)
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