The following code will work if I run it from the ArcMap Python dialog, or from a GP tool, but not from PyScripter IDE:

import arcpy

mxd = arcpy.mapping.MapDocument(r"c:\temp\<my_mxd_name>.mxd")
layerList = arcpy.mapping.ListLayers(mxd)

for lyr in layerList:
  print "Layer = " + lyr.name

  #Get all fields in the layer
  fList = arcpy.Describe(lyr).fields
  for f in fList:
    print "Field = "  + f.baseName

When I run the exact same code from PyScripter, it fails on arcpy.Describe(lyr).fields with the error IOError: "<TOC_layer_name>" does not exist, even though it does exist in the referenced MXD table of contents. When PyScripter fails it opens up the ESRI file _base.py to the describe function. I am assuming there must be something different about describing a layer in ArcMap vs calling it directly from Python, but cannot figure out what it could be.

How do I properly use the arcpy.Describe() function on a layer when I am not running the script directly from ArcMap?

I have figured out that if I use arcpy.Describe(lyr.dataSource) it functions properly, but only returns the fields from the source data layer. However, since I'm trying to read fields from a joined table (which does work in the original code above when running from AcrMap), I really need to see the fields from all joined tables, not just the data source.

  • Have you tried incorporating the Add Join tool in your code? However, it would be nicer if the MapDocument and Layer objects were join-aware.
    – PolyGeo
    Jul 23, 2013 at 22:48
  • @PolyGeo, the layers are already joined via the UI, and this is a process automation task where I am just checking for existence of joined fields, not adding the join. Jul 23, 2013 at 22:49
  • Might have to be an ArcGIS Idea (for ArcGIS Professional) I think - if you go that way post a link and I will vote for it.
    – PolyGeo
    Jul 23, 2013 at 22:51
  • 1
    I just made a little test and ran the exact code that you put up there (without lyr.dataSource) and it worked for me. I ran it inside PyScripter and it printed my two layers that I joined and for one of them it printed all its fields plus the fields from the join. I did create the join inside ArcMap and then saved and closed the MXD. Are you creating the join inside ArcMap or through Python code outside?
    – ianbroad
    Jul 25, 2013 at 16:40
  • @I-B, I followed all the same steps as you, with no luck. Created the join in ArcMap, using right-click on layer-> Joins & Relates-> Join GUI with the "Keep all records" option, and saving/closing ArcMap. What version and service pack of ArcGIS are you running? Jul 25, 2013 at 17:27

1 Answer 1


Sadly, I am going to have to chalk this one up to a bug that was introduced in Arc 10.0 SP1 or SP2.

From what I can tell, this bug was resolved in 10.1 (as @I-B suggested, and I confirmed). Why? Because I uninstalled SP2, SP3, SP4, & SP5 (I never installed SP1) all the way back to Arc 10.0 (no service packs), where the same exact code was finally able to run again.

To resolve this, I added the following code:

    desc = arcpy.Describe(lyr)
    #This is a workaround for Arc10.0 bug
    #This points back to the path/filename of the layer
    desc = arcpy.Describe(lyr.dataSource)

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