I have the following function that gets a source and a modified strings, and bolds the changed words in it.

def appendBoldChanges(s1, s2):"Adds <b></b> tags to words that are changed"l1 = s1.split(' ')l2 = s2.split(' ')for i, val in enumerate(l1):if l1[i].lower() != l2[i].lower():s2 = s2.replace(l2[i], "<b>%s</b>" % l2[i])return s2
print appendBoldChanges("britney spirs", "britney spears") # returns britney <b>spears</b>

It works fine on strings with the same word count, but fails with different word counts, as sora iro days and sorairo days.

How can I take the spaced into consideration?

You could use difflib, and do it like this:

from difflib import Differdef appendBoldChanges(s1, s2):"Adds <b></b> tags to words that are changed"l1 = s1.split(' ')l2 = s2.split(' ')dif = list(Differ().compare(l1, l2))return " ".join(['<b>'+i[2:]+'</b>' if i[:1] == '+' else i[2:] for i in dif if not i[:1] in '-?'])print appendBoldChanges("britney spirs", "britney sprears")
print appendBoldChanges("sora iro days", "sorairo days")
#Output:
britney <b>sprears</b>
<b>sorairo</b> days