Python confusion

...at least for a newbie.

• lists: L = ['a', 'b', 'this is another element', 1, [1, 2, 3]]. Can do a L.append(), len(L), L.pop(), etc
  
• tuples: T = 1, 2, 3, 4, 'this is a tuple element'. Or T1 = () for an empty tuple, T2= 'one element tuple', . None of the functions listed above apply.
  
• sets: S = {1, 2, 3, 'set element'}. The items must be unique and set functions are available. S = set(L) converts list to set (removes duplicates in the process).