A Python3 MutableSequence, polymorphic singly linked list (for educational purposes) offering:

• creation with any number of items: s1 = Sll(0, 1, 2); s2 = Sll()

• numeric indexing of list items (0 or greater): s[0]

• iteration: for item in s1

• emptiness and boolean truthiness testing:
  s1.is_empty() returns False; s2.is_empty() returns True
s1 and not s2 returns True

• containment for finding if an item is in the list: if 2 in s1: print("found it")

• concatenation with '+': s1 + s2

• duplication and extending with '*': s1 * 2 returns Sll[0, 1, 2, 0, 1, 2]

• prepending and extending by any number of items:
  s1.prepend(3, 4) returns Sll[3, 4, 0, 1, 2]
s1.extend(5, 6) returns Sll[3, 4, 0, 1, 2, 5, 6]
s2.extend(3, 4) returns Sll[3, 4]

• appending single items: s2.append(5) returns Sll[3, 4, 5]

• finding an item and returning its sublist: s2.find(4) returns Sll[4, 5]

• counting item occurrences: s2.count(3) returns 1

• list or item deletion via 'del': del s1[2] or del s1

• access to last item or sublist:
  s2.last() returns 5
s2.last_sublist() returns Sll[5]

• conversion to a standard Python list of items: s2.items() returns [3, 4, 5]

• a full set of self-tests: Sll.test().