Hi Rajiv, Premise 1: Every time you *create* an object, it has a new id. Examples: >>> a = list () >>> b = list() >>> id(a), id(b) (140688241027768, 140688241027840)
>>> p = dict() >>> q = dict() >>> id(p), id(q) (140688241071280, 140688241071560) >>> x = iter(range(10)) >>> y = iter(range(10)) >>> id(x), id(y) (140688241024208, 140688241024272) Premise 2: Every time you associate the same object to multiple names, they all will point to same object. Examples: >>> a = list() >>> b = a >>> id(a), id(b) (140688241028200, 140688241028200) >>> p = dict() >>> q = p >>> id(p), id(q) (140688241071840, 140688241071840) >>> x = iter(range(10)) >>> y = x >>> id(x), id(y) (140688241024464, 140688241024464) In your problem, Case 1 and Case 3 follow the pattern [an_object]*3 which results in [an_object, an_object, an_object] where as Case 2 follows the pattern [create_object(), create_object(), create_object()] Note that in Case 3, iter(x) is evaluated before multiplying. Regards, Abdul Muneer -- Follow me on Twitter: @abdulmuneer <http://twitter.com/#%21/abdulmuneer> On Thu, Dec 11, 2014 at 4:21 PM, Rajiv Subramanian M <rajiv.m1...@gmail.com> wrote: > > Hi Shreyas, > > Thanks for your answer. > For the questions numered 2 and 3 do you have any thoughts? > > 2. Is there any other possibility or way (like * operator does here) by > which we can obtain the same result as in CASE 1? > 3. Does only list and listiterators objects can behave this way? what other > python datatypes can behave this way? > > > > On Thu, Dec 11, 2014 at 4:14 PM, Kulkarni, Shreyas <shy...@gmail.com> > wrote: > > > When you call iter(x) it returns you a listiterator object. Every time > you > > call iter(x) it *creates* a new listiterator and returns it back. The > > differences you are seeing in your cases are not because of how lists of > > list operators work, but because of how they are called. > > > > In case-1, iter(x) gets called once, and the same returned object is used > > when you do '* 3' on the list. > > In case-2, you are calling iter(x) three times, so naturally you get 3 > > different iterators - three different objects with different base > addresses > > in memory. > > > > Case-3 is how python typically works - when you do an assignment, python > > doesn't create a copy, but a reference. So when you say y = [x] * 3; y is > > essentially a list with 3 references to the same memory location pointed > to > > by x. So when you update one value, all three refs point to the same > > location, and hence you are seeing what you are seeing. > > If you want deep copy instead of shallow one with references, take a look > > at 'copy' module and copy.deepcopy method in particular. > > > > shreyas > > > > > > On Dec 11, 2014, at 3:58 PM, Rajiv Subramanian M <rajiv.m1...@gmail.com> > > wrote: > > > > > Hello Group, > > > > > > I am Rajiv, Python/Django developer in a startup, Bangalore. Today > when I > > > experimenting with python I came across the following > > > > > > CASE 1: > > > > > >>>> x = range(10) > > > > > >>>> [iter(x)] * 3 > > > > > > [<listiterator object at 0x7f6aa5594850>, > > > > > > <listiterator object at 0x7f6aa5594850>, > > > > > > <listiterator object at 0x7f6aa5594850>] > > > > > > > > > Thing to Note: > > > > > > Here all the 3 listiterator object in the list is actually a same > single > > > instance residing at the memory location "0x7f6aa5594850" > > > > > > > > > CASE 2: > > > > > >>>> [iter(x), iter(x), iter(x)] > > > > > > [<listiterator object at 0x7f6aa5594890>, > > > > > > <listiterator object at 0x7f6aa55948d0>, > > > > > > <listiterator object at 0x7f6aa5594910>] > > > > > > > > > Thing to Note: > > > In this case literally I created called the iter(x) for 3 times so it > > > created 3 different listiterator object. > > > > > > CASE 3: > > >>>> x = [1, 2, 3] > > >>>> y = [x] * 3 > > >>>> y > > > [[1, 2, 3], [1, 2, 3], [1, 2, 3]] > > >>>> y[0][0] = 5 > > >>>> y > > > [[5, 2, 3], [5, 2, 3], [5, 2, 3]] > > >>>> x > > > [5, 2, 3] > > > > > > Things to Note: > > > As like in first case, here the list objects inside the list y are not > > the > > > duplicates of x but the x itself > > > > > > > > > Question: > > > 1. How in the first case i.e [iter(x)] * 3 creates a list of 3 but > the > > > same objects? > > > 2. Is there any other possibility or way (like * operator does here) by > > > which we can obtain the same result as in CASE 1? > > > 3. Does only list and listiterators objects can behave this way? what > > other > > > python datatypes can behave this way? > > > > > > ~ Regards > > > Rajiv M > > > _______________________________________________ > > > BangPypers mailing list > > > BangPypers@python.org > > > https://mail.python.org/mailman/listinfo/bangpypers > > > > _______________________________________________ > > BangPypers mailing list > > BangPypers@python.org > > https://mail.python.org/mailman/listinfo/bangpypers > > > > > > -- > > [image: --] > Rajiv Subramanian M > [image: http://]about.me/rajiv.m1991 > <http://about.me/rajiv.m1991?promo=email_sig> > _______________________________________________ > BangPypers mailing list > BangPypers@python.org > https://mail.python.org/mailman/listinfo/bangpypers > _______________________________________________ BangPypers mailing list BangPypers@python.org https://mail.python.org/mailman/listinfo/bangpypers
