It's part of an evolution.

Originally, Python items() built a real list of tuples and returned that. That could potentially take a lot of extra memory.

Then, generators were introduced to the language in general, and that method was reimplemented as an iterator-generator method named iteritems(). The original remains for backwards compatibility.

One of Python 3’s changes is that items() now return views, and a list is never fully built. The iteritems() method is also gone, since items() in Python 3 works like viewitems() in Python 2.7.

dict.items() returns a list of 2-tuples ([(key, value), (key, value), ...]), whereas dict.iteritems() is a generator that yields 2-tuples. The former takes more space and time initially, but accessing each element is fast, whereas the second takes less space and time initially, but a bit more time in generating each element.

In Py2.x

The commands dict.items(), dict.keys() and dict.values() return a copy of the dictionary's list of (k, v) pair, keys and values.This could take a lot of memory if the copied list is very large.

The commands dict.iteritems(), dict.iterkeys() and dict.itervalues() return an iterator over the dictionary’s (k, v) pair, keys and values.

The commands dict.viewitems(), dict.viewkeys() and dict.viewvalues() return the view objects, which can reflect the dictionary's changes.(I.e. if you del an item or add a (k,v) pair in the dictionary, the view object can automatically change at the same time.)

$ python2.7>>> d = {'one':1, 'two':2}>>> type(d.items())<type 'list'>>>> type(d.keys())<type 'list'>>>> >>> >>> type(d.iteritems())<type 'dictionary-itemiterator'>>>> type(d.iterkeys())<type 'dictionary-keyiterator'>>>> >>> >>> type(d.viewitems())<type 'dict_items'>>>> type(d.viewkeys())<type 'dict_keys'>

While in Py3.x

In Py3.x, things are more clean, since there are only dict.items(), dict.keys() and dict.values() available, which return the view objects just as dict.viewitems() in Py2.x did.

But

Just as @lvc noted, view object isn't the same as iterator, so if you want to return an iterator in Py3.x, you could use iter(dictview) :

$ python3.3>>> d = {'one':'1', 'two':'2'}>>> type(d.items())<class 'dict_items'>>>>>>> type(d.keys())<class 'dict_keys'>>>>>>>>>> ii = iter(d.items())>>> type(ii)<class 'dict_itemiterator'>>>>>>> ik = iter(d.keys())>>> type(ik)<class 'dict_keyiterator'>