Working with text in Python

Position counting starts at zero, so word[0] is the first character. A negative index counts back from the end, so word[-1] is the last one and word[-2] would be the character before it. That saves writing word[len(word) - 1]. Indexing past the end of a string does raise an error, but a negative index that stays within the string is fine, so the option that claims an error is wrong.

Strings in Python cannot be changed in place. A method such as lower returns a new string and leaves the one it was called on exactly as it was, so printing line after calling line.lower() still shows Admin:user. To keep the cleaned value you have to store it, as in clean = line.lower(). Nothing raises an error, which is what makes the mistake quiet: the code runs and only the result is wrong.

join is the reverse of split, and it is called on the separator rather than on the list. You write "-".join(parts), so the hyphen is what does the joining and the list is what gets joined. parts.join("-") reads naturally and looks plausible, but a list has no join method, so it raises an error. split does the opposite job, breaking one string into a list, so it cannot join anything.

A slice includes its start and excludes its end, so code[4:8] gives positions 4, 5, 6 and 7, which are the four digits of the year, 2026. The hyphen sits at position 3, so the year starts at 4. The prefix is code[:3] and the last four characters are code[-4:]. A slice that runs past the end of the string simply stops, so it does not raise an error, unlike indexing a single position that is out of range.