Populates the next highest numbers for a feature class based on the previous max

autoIncrementUniqueID.py

import arcpy

lyr = arcpy.management.MakeFeatureLayer(r"C:\Some\Arb\Folder\work.gdb\ftr_wrp_Pipe")
field = "GIS_ID"

def getWhereQry(exp):
    return """ "{0}" IS {1} """.format(field, exp)

# Gets the previous highest GIS ID value. Set n = 0 for new feature class
n = max(int(row[0].split("-")[1]) for row in arcpy.da.SearchCursor(lyr, field, where_clause=getWhereQry("NOT NULL")))
numstring = "00000"
prefix = "WRP"
sep = "-"

# Populate blank GIS IDs with new values
with arcpy.da.UpdateCursor(lyr, field, where_clause=getWhereQry("NULL")) as cursor:
    for row in cursor:
        n += 1
        numstring_length = len(numstring) - len(str(n))
        row[0] = prefix + sep + numstring[:numstring_length] + str(n)
        cursor.updateRow(row)