30 Days Of Python

https://www.geeksforgeeks.org/python/python-string-strip/ The strip method removes by default whitespaces at the beginning and end of a string. If you specify a string as you did, it will remove the characters at the beginning and end that belong to that set of characters. In your case, you can see it as if you passed the set (n, o, t, h) to be removed, so if it finds one of those characters at the beginning or end, it will remove them, hence your result is 'irty days of py'