How to write a black-box test case to test a sort function in python?

Recently I have been taking python courses, but still have many problems. I don't know how to write a test case. Here is the question.

Assume that you are repsonsible for testing a sort function, which is being provided by a third party. The implementation should be consistent with the following specification:

== Specification of sort ==

• takes an Iterable as a parameter that contains comparable elements
• sorts all contained elements without changing the original Iterable
• elements should be sorted in ascending order (i.e., smallest first)
• always returns a new List that contains the sorted elements
• If parameter is None or non-iterable, return None.

The implementaion can assume that all elements of a provided iterable are comparable data types (e.g., int, float, string). This means that relational operations like '<', '>', etc. are defined. The behavior for data types that are not comparable is not defined and they do not need to be handled.

The test case's structure looks like:

 class SortTests(TestCase):
     pass

Thanks.