Several methods of the dictionary class that return a list under 2.5 and before now return a view. Under Pytho <=2.5, the value returned reflected the dictionary items at the time the method was called. If the dictionary changed immediately afterward, the list did not change. Views allow for dynamic assignment of dictionary items (or their sundry aspects). So, the values refleced in the calls will change with the dictionary. For most programming, this difference will not matter greatly. For example, see the following dictionary containing some of this year's Nobel Prize winners:
>>> dict = {"1953":"Paul Krugman", "1921":"Yoichiro Nambu", "1944":"Makoto Kobayashi", "1940":"Toshihide Maskawa", "1928":"Osamu Shimomura", "1947":"Martin Chalfie", "1952":"Roger Y. Tsien"}
>>> keys = sorted(dict)
>>> for year in keys:
... print(dict[year], year, sep=": ")
...
Yoichiro Nambu: 1921
Osamu Shimomura: 1928
Toshihide Maskawa: 1940
Makoto Kobayashi: 1944
Martin Chalfie: 1947
Roger Y. Tsien: 1952
Paul Krugman: 1953
>>> dict["1937"]="Martti Ahtisaari"
>>> for year in keys:
... print(dict[year], year, sep=": ")
...
Yoichiro Nambu: 1921
Osamu Shimomura: 1928
Toshihide Maskawa: 1940
Makoto Kobayashi: 1944
Martin Chalfie: 1947
Roger Y. Tsien: 1952
Paul Krugman: 1953
Note that we have not re-assigned the new list. Once we re-assign keys, however, the output of the loop changes.
>>> keys = sorted(dict)
>>> for year in keys:
... print(dict[year], year, sep=": ")
...
Yoichiro Nambu: 1921
Osamu Shimomura: 1928
Martti Ahtisaari: 1937
Toshihide Maskawa: 1940
Makoto Kobayashi: 1944
Martin Chalfie: 1947
Roger Y. Tsien: 1952
Paul Krugman: 1953
Dictionary methods that are now returning views instead of lists are dict.keys(), dict.items() and dict.values(). Also, some are no longer supported: dict.iterkeys(), dict.iteritems(), and dict.itervalues().
