What is the difference between dict.items() and dict.iteritems() in Python2?

Question

Are there any applicable differences between dict.items() and dict.iteritems()?

From the Python docs:

dict.items(): Return a copy of the dictionary’s list of (key, value) pairs.

dict.iteritems(): Return an iterator over the dictionary’s (key, value) pairs.

If I run the code below, each seems to return a reference to the same object. Are there any subtle differences that I am missing?

#!/usr/bin/python  d={1:'one',2:'two',3:'three'} print 'd.items():' for k,v in d.items():    if d[k] is v: print '\tthey are the same object'     else: print '\tthey are different'  print 'd.iteritems():'    for k,v in d.iteritems():    if d[k] is v: print '\tthey are the same object'     else: print '\tthey are different'    

Output:

d.items():     they are the same object     they are the same object     they are the same object d.iteritems():     they are the same object     they are the same object     they are the same object 

Answer 1

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.

Answer 2

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.