In a python-checkins message, Raymond stated: Raymond> Replace list of constants with tuples of constants. I understand the motivation here (the peephole optimizer can convert a tuple of constants into a single constant that need not be constructed over and over), but is the effort worth the cost of changing the logical nature of the data structures used? If lists are conceptually like vectors or arrays in other languages and tuples are like C structs or Pascal records, then by converting from list to tuple form you've somehow muddied the data structure water just to take advantage of tuples' immutability. Wouldn't it be better to have the peephole optimizer recognize the throwaway nature of lists in these contexts: for elt in [1, 2, 4, 8, 16]: ... if foo in [list, tuple]: ... (anywhere a list of constants immediately follows the "in" or "not in" keywords) and convert them into constants? The cases you converted all matched that usage. Skip