arcpy - Calculate Field - getting the code value but not the description

If STREETNM value are blank or _ then I want to replace them with the value of ST_DESIG. My code below returned the code value of ST_DESIG but not the description.

How can I get the description value of the ST_DESIG. For example, "Divided Major Highway - Class II" NOT 1.

import arcpy
Streets_Centerlines = r"G:\xStreetNew\Test.gdb\Streets_Centerlines"
expression = "replace(!STREETNM!)"
codeblock = """
def replace(STREETNM,ST_DESIG):
 if STREETNM.lstrip().rstrip() == '' or STREETNM.lstrip().rstrip() ==    '_':
      return ST_DESIG
 else:
      return STREETNM

"""
arcpy.CalculateField_management(Streets_Centerlines, "STREETNM", "replace(!STREETNM!,!ST_DESIG!)", "PYTHON", codeblock)