def f(sentence): ls = list(sentence) for letter in ls: if not letter.istitle(): ls.remove(letter) return ''.join(ls) assert f('XYZ LittleRedRidingHood LiTTleBIGGeXEiT fault') == 'XYZLtRRdnHodLTTBIGGeXET fult'
assert f(??) == 'XYZLtRRdnHodLTTBIGGeXET fult'
assert f('XYZ LittleRedRidingHood LiTTleBIGGeXEiT fault') == ??