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

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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   gt  gt  gt  d     one  1   two  2   gt  gt  gt  type d items     lt type  list  gt   gt  gt  gt  type d keys     lt type  list  gt   gt  gt  gt    gt  gt  gt    gt  gt  gt  type d iteritems     lt type  dictionary-itemiterator  gt   gt  gt  gt  type d iterkeys     lt type  dictionary-keyiterator  gt   gt  gt  gt    gt  gt  gt    gt  gt  gt  type d viewitems     lt type  dict items  gt   gt  gt  gt  type d viewkeys     lt type  dict keys  gt    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   gt  gt  gt  d     one   1    two   2    gt  gt  gt  type d items     lt class  dict items  gt   gt  gt  gt   gt  gt  gt  type d keys     lt class  dict keys  gt   gt  gt  gt   gt  gt  gt   gt  gt  gt  ii   iter d items     gt  gt  gt  type ii   lt class  dict itemiterator  gt   gt  gt  gt   gt  gt  gt  ik   iter d keys     gt  gt  gt  type ik   lt class  dict keyiterator  gt

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dict iteritems   in python 2 is equivalent to dict items   in python 3

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If you want a way to iterate the item pairs of a dictionary that works with both Python 2 and 3  try something like this   DICT ITER ITEMS    lambda d  d iteritems    if hasattr dict   iteritems   else  lambda d  iter d items       Use it like this   for key  value in DICT ITER ITEMS myDict         Do something with  key  and or  value

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You asked   Are there any applicable differences between dict items   and dict iteritems     This may help  for Python 2 x     gt  gt  gt  d  1  one  2  two  3  three    gt  gt  gt  type d items     lt type  list  gt   gt  gt  gt  type d iteritems     lt type  dictionary-itemiterator  gt    You can see that d items   returns a list of tuples of the key  value pairs and d iteritems   returns a dictionary-itemiterator   As a list  d items   is slice-able    gt  gt  gt  l1 d items   0   gt  gt  gt  l1  1   one       an unordered value    But would not have an   iter   method    gt  gt  gt  next d items    Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  list object is not an iterator   As an iterator  d iteritems   is not slice-able    gt  gt  gt  i1 d iteritems   0  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError   dictionary-itemiterator  object is not subscriptable   But does have   iter      gt  gt  gt  next d iteritems     1   one                   an unordered value    So the items themselves are same -- the container delivering the items are different  One is a list  the other an iterator  depending on the Python version       So the applicable differences between dict items   and dict iteritems   are the same as the applicable differences between a list and an iterator

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If you have   dict    key1 value1  key2 value2  key3 value3       In Python 2  dict items   copies each tuples and returns the list of tuples in dictionary i e    key1 value1    key2 value2          Implications are that the whole dictionary is copied to new list containing tuples  dict    i  i   2 for i in xrange 10000000       Slow and memory hungry  for key  value in dict items        print key     value    dict iteritems   returns the dictionary item iterator  The value of the item returned is also the same i e   key1 value1    key2 value2        but this is not a list  This is only dictionary item iterator object  That means less memory usage  50  less     Lists as mutable snapshots  d items   - gt  list d items     Iterator objects  d iteritems   - gt  iter d items      The tuples are the same  You compared tuples in each so you get same   dict    i  i   2 for i in xrange 10000000       More memory efficient  for key  value in dict iteritems        print key     value    In Python 3  dict items   returns iterator object  dict iteritems   is removed so there is no more issue

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dict iteritems is gone in Python3 x So use iter dict items    to get the same output and memory alocation

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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

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dict iteritems    gives you an iterator  You may use the iterator in other patterns outside of the loop   student     name    Daniel    student id   2222   for key value in student items        print key value     student id   2222    name    Daniel    for key value in student iteritems        print key value     student id   2222    name    Daniel    studentIterator   student iteritems    print studentIterator next      student id   2222   print studentIterator next      name    Daniel

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dict items   return list of tuples  and dict iteritems   return iterator object of tuple in dictionary as  key value   The tuples are the same  but container is different    dict items   basically copies all dictionary into list  Try using following code to compare the execution times of the dict items   and dict iteritems    You will see the difference   import timeit  d    i i 2 for i in xrange 10000000     start   timeit default timer    more memory intensive for key value in d items        tmp   key   value  do something like print t1   timeit default timer   - start  start   timeit default timer   for key value in d iteritems     less memory intensive     tmp   key   value t2   timeit default timer   - start   Output in my machine   Time with d items    9 04773592949 Time with d iteritems    2 17707300186   This clearly shows that dictionary iteritems   is much more efficient

User · Answer

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

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

In Py2.x

While in Py3.x

But

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