Given original info:

Original Data: ['a', 'b', 'c', 'c', 'd', 'e']
Original Codes: [0 1 2 2 3 4] returned by pd.factorize(original_data)

new_data = ['a', 'd', 'e', 'k']
new_codes, new_uniques = pd.factorize(new_data)
new_codes1, new_uniques1 = pd.factorize(new_data, original_factorization=(original_uniques, original_codes))

new_codes will be 0123 (functionality of existing factorize function did not change)
new_codes1 will be 0345 (we can pass in optional parameter)