Adds a new field and assigns an existing domain to a feature class, or assigns the domain only if the field exists.

assignExistingDomainToField.py

#
# @date 25/02/2015
# @author Cindy Williams
#
# Adds a new field to feature classes in a file GDB
# and assigns an existing domain to it. If the field
# exists, only the domain is assigned.
#
# For use in the Python window in ArcCatalog.
#

import arcpy
 
arcpy.env.workspace = r"C:\Some\Arb\Folder\work.gdb"

# Input variables
cvd = "cvd_District" # Name of existing coded value domain
field_nam = "District" # Name of new field to be added to feature class

for fc in arcpy.ListFeatureClasses():
    try:
        arcpy.management.AddField(in_table=fc,
                                  field_name=field_nam,
                                  field_type="SHORT",
                                  field_alias=field_nam,
                                  field_domain=cvd)
        print("Added field and assigned domain to " + fc)
    except:
        print("Field exists in " + fc)
        arcpy.management.AssignDomainToField(in_table=fc,
                                             field_name=field_nam,
                                             domain_name=cvd)
        print("Domain assigned to " + fc)

print("Script complete.")