The last part of the precondition for

str$\_$to$\_$seconds is that the string is in ISO $8601$

format.

This was the purpose of the function iso$\_8601.$ It

returns True if the string is in the right format and

False if it is not. That means you can use it as part

an assert statement.

Add on last assert statement to str$\_$to$\_$seconds

that calls

Again, error messages are

optional and are not required. To test your assert

statement, you will need to call the function

in the interactive shell. Calls like

str$\_$to$\_$seconds$(\text{'}1$:$301$:$12\text{'})$ or

str$\_$to$\_$seconds$(\text{'}12$:aa:$12\text{'})$ should crash with an

AssertionError if you did it properly.