much like the keys of valueless dictionarythey don' map keys to valuesbut can often be used like dictionaries for fast lookups when there is no associated valueespecially in search routinesd { ['state 'true 'state in true set( add('state ''state in true visited-state dictionary samebut with sets watch for rehash of this record representation thread in the next where we'll see how tuples and named tuples compare to dictionaries in this roleas well as in where we'll learn how user-defined classes factor into this picturecombining both data and logic to process it dictionary changes in python and this has so far focused on dictionary basics that span releasesbut the dictionary' functionality has mutated in python if you are using python codeyou may come across some dictionary tools that either behave differently or are missing altogether in moreover coders have access to additional dictionary tools not available in xapart from two back-ports to specificallydictionaries in python xsupport new dictionary comprehension expressiona close cousin to list and set comprehensions return set-like iterable views instead of lists for the methods keysd valuesand items require new coding styles for scanning by sorted keysbecause of the prior point no longer support relative magnitude comparisons directly--compare manually instead no longer have the has_key method--the in membership test is used instead as later back-ports from xdictionaries in python (but not earlier in )support item in the prior list--dictionary comprehensions--as direct back-port from support item in the prior list--set-like iterable views--but do so with special method names viewkeysd viewvaluesd viewitems)their nonview methods return lists as before lists and dictionaries