I use something like this, that will check if a required domain exits, if the domain exists, check for required values, check if a field exists, and assign the domain to the field. It will also add things if they don't exist at all. Can be easily modified to meet your requirement..
try:
# 2 Create COUNTY field and TLU_COUNTY domain if user checked box
in_Field = "COUNTY"
if str(isCOUNTYchecked) == 'true':
TLU_COUNTYDict = {"Swain":"Swain", "Blount": "Blount", "Sevier": "Sevier", "Cocke": "Cocke", "Haywood": "Haywood", "Graham": "Graham", "Monroe": "Monroe"}
domains = arcpy.da.ListDomains(gdb_name)
domain_names = [domain.name for domain in domains]
if 'TLU_COUNTY' in domain_names :
arcpy.AddWarning("TLU_COUNTY already exists")
for domain in domains:
if domain.name == 'TLU_COUNTY':
values = [cv for cv in domain.codedValues]
if not set(set(["Swain", "Blount", "Sevier", "Cocke", "Haywood", "Graham", "Monroe"])).issubset(values):
arcpy.AddWarning("TLU_COUNTY is missing a coded value pair.")
for code in TLU_COUNTYDict:
arcpy.AddCodedValueToDomain_management(gdb_name, "TLU_COUNTY", code, TLU_COUNTYDict
)
arcpy.AddWarning("But Sasquatch took care of it.")
else:
arcpy.SetParameterAsText(10, gdb_name)
arcpy.CreateDomain_management(gdb_name, "TLU_COUNTY", "County feature occurs in", "TEXT", "CODED")
arcpy.AddMessage ("Sasquatch created the TLU_COUNTY domain")
for code in TLU_COUNTYDict:
arcpy.AddCodedValueToDomain_management(gdb_name, "TLU_COUNTY", code, TLU_COUNTYDict)
arcpy.AddMessage ("Sasquatch added coded domain values to the TLU_COUNTY domain")
if len(arcpy.ListFields(fc_name,in_Field))>0:
arcpy.AddWarning (in_Field+" exits already")
else:
arcpy.AddField_management(fc_name, in_Field, "TEXT", "", "", 30, in_Field, "NULLABLE", "NON_REQUIRED", "TLU_COUNTY")
arcpy.AddMessage ("Sasquatch created the "+in_Field+" field and the attribute domain is TLU_COUNTY")
arcpy.AssignDefaultToField_management(fc_name, in_Field)
else:
arcpy.AddWarning(in_Field+" field will not be created")
