Some words which end with -us that have plural forms with -i are giving incorrect results.

>>> word_list = ['focus','cactus','fungus','nucleus','syllabus']
>>> for w in word_list:
...     print(pluralize(w))
...
foci
cactuss
fungi
nucleuss
syllabuss

Should be cacti, nuclei and syllabi respectively. There are enough words of this sort that can form a group. The singularize function also converts incorrectly.

>>> singularize('fungi')
'fungi'