Adds a new field using a numpy array to layers in a map document.

addFieldWithExtendTable.py

'''
@date 12/10/2015
@author Cindy Williams

Adds a new field to layers in a map document, based
on a current field.

For use in the Python window in ArcMap.
'''

import arcpy
import numpy

mxd = arcpy.mapping.MapDocument("CURRENT")
lyrs = arcpy.mapping.ListLayers(mxd)

# Create a numpy array with a link oid field and the fields to be added
narray = numpy.array([], numpy.dtype([('objid', numpy.int), ('GIS_ID', '|S10'),]))

for lyr in lyrs:
  if lyr.isFeatureLayer:
    # Find the current field with the values 
    field = [field.name for field in arcpy.ListFields(lyr, "GIS_*")][0]
    # Prevent the tool from failing
    if field != "GIS_ID":
    # Add the field
      arcpy.da.ExtendTable(lyr,"OID@", narray, "objid")
      # Copy the field values to the new field
      with arcpy.da.UpdateCursor(lyr, ("GIS_ID", field)) as cursor:
        for row in cursor:
          row[0] = row[1]
          cursor.updateRow(row)