Late Library Books

A library lends books for 14 days. Its records give, for each loan, the book's title, the date it was borrowed and the date it came back. A book that hasn't come back yet has None as its return date.

Write days_late(titles, borrowed, returned, today). The three lists line up position by position. All dates are text in the form "YYYY-MM-DD", and so is today.

• A loan ends on its return date. If the book is still out (None), count it as ending on the date given by today.
• Days late = (days from borrowing to the end of the loan) − 14.

Return a dictionary mapping each title that is more than 0 days late to how many days late it is, as a whole number, in the same order as the input. Titles are all different. If nothing is late, return an empty dictionary.