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Pure Functions Practice

Datetimes are infamously a pain in the neck for programming. One of the easier items on the long list of problems is the order of the year, month, and day in a calendar date. Most of the world uses the day-month-year format, but some insane countries use month-day-year because they want everyone else to be miserable.

Assignment

Refactor the sort_dates function. It currently uses .sort(), which modifies the list in-place (an impure operation). We want to change it to use sorted() with a custom helper function, to sort the dates chronologically without modifying the original list.

Define a helper function named format_date outside of sort_dates.
1. It should accept a single string (a date) in "MM-DD-YYYY" format (e.g. "10-14-1995").
2. Inside the function, split the input string and rearrange it to return a new string in YYYYMMDD format. For example: 10-05-2023 → 20231005.
Update the sort_dates function.
1. It should accept a list of date strings.
2. It should return a new sorted list using the built-in sorted() function.
3. Pass your format_date function as the key parameter to sorted().

When you pass key=format_date, Python runs your helper function on every item to figure out the sort order (using the YYYYMMDD format), but it puts the original dates (MM-DD-YYYY) into the final list.