In Python what is the difference between append and

Question

What is the difference between   some list1      some list1 append  something     and  some list2      some list2      something

User · Answer

In the example you gave  there is no difference  in terms of output  between append and      But there is a difference between append and    which the question originally asked about     gt  gt  gt  a       gt  gt  gt  id a  11814312  gt  gt  gt  a append  hello    gt  gt  gt  id a  11814312   gt  gt  gt  b       gt  gt  gt  id b  11828720  gt  gt  gt  c   b     hello    gt  gt  gt  id c  11833752  gt  gt  gt  b      hello    gt  gt  gt  id b  11828720   As you can see  append and    have the same result  they add the item to the list  without producing a new list   Using   adds the two lists and produces a new list

User · Answer

The append   method adds a single item to the existing list  some list1      some list1 append  something     So here the some list1 will get modified   Updated   Whereas using   to combine the elements of lists  more than one element  in the existing list similar to the extend  as corrected by Flux    some list2      some list2      something     So here the some list2 and   something   are the two lists that are combined

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In addition to the aspects described in the other answers  append and     have very different behaviors when you re trying to build a list of lists    gt  gt  gt  list1   1 2   3 4    gt  gt  gt  list2  5 6   gt  gt  gt  list3 list1 list2  gt  gt  gt  list3   1  2    3  4   5  6   gt  gt  gt  list1 append list2   gt  gt  gt  list1   1  2    3  4    5  6     list1   5   6   adds  5  and  6  to the list1 as individual elements   list1 append   5   6    adds the list   5   6   to the list1 as a single element

User · Answer

let s take an example first  list1  1 2 3 4  list2 list1      that means they points to same object   if we do  list1 list1  5     it will create a new object of list print list1        output  1 2 3 4 5   print list2        output  1 2 3 4   but if we append  then  list1 append 5      no new object of list created print list1        output  1 2 3 4 5   print list2        output  1 2 3 4 5   extend list  also do the same work as append it just append a list instead of a  single variable

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The performance tests here are not correct    You shouldn t run the profile only once  If comparing append vs        number of times you should declare append as a local function  time results are different on different python versions  64 and 32 bit   e g                    timeit Timer  for i in xrange 100   app i     s        app   s append   timeit              good tests can be found here  http   markandclick com 1 post 2012 01 python-list-append-vs html

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As of today and Python 3 6  the results provided by  Constantine are no longer the same  Python 3 6 10  Anaconda  Inc    default  May  8 2020  02 54 21    GCC 7 3 0  on linux Type  quot help quot    quot copyright quot    quot credits quot  or  quot license quot  for more information   gt  gt  gt  import timeit  gt  gt  gt  timeit Timer  s append  quot something quot      s        timeit   0 0447923709944007  gt  gt  gt  timeit Timer  s      quot something quot      s        timeit   0 04335783299757168  It seems that append and    now have an equal performance  whereas the compilation differences haven t changed at all   gt  gt  gt  import dis  gt  gt  gt  dis dis compile  quot s       s append  spam   quot        exec      1           0 BUILD LIST               0               2 STORE NAME               0  s                4 LOAD NAME                0  s                6 LOAD ATTR                1  append                8 LOAD CONST               0   spam                10 CALL FUNCTION            1              12 POP TOP              14 LOAD CONST               1  None               16 RETURN VALUE  gt  gt  gt  dis dis compile  quot s       s      spam   quot        exec      1           0 BUILD LIST               0               2 STORE NAME               0  s                4 LOAD NAME                0  s                6 LOAD CONST               0   spam                 8 BUILD LIST               1              10 INPLACE ADD              12 STORE NAME               0  s               14 LOAD CONST               1  None               16 RETURN VALUE

User · Answer

some list2      something     is actually    some list2 extend   something      for one value  there is no difference  Documentation states  that      s append x      same as s len s  len s      x     s extend x      same as s len s  len s     x   Thus obviously s append x  is same as s extend  x

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is an assignment  When you use it you re really saying    some list2  some list2   something       Assignments involve rebinding  so   l      def a1 x       l append x    works  def a2 x       l  l  x    assign to l  makes l local                so attempt to read l for addition gives UnboundLocalError  def a3 x       l    x     fails for the same reason   The    operator should also normally create a new list object like list list normally does    gt  gt  gt  l1      gt  gt  gt  l2  l1   gt  gt  gt  l1 append  x    gt  gt  gt  l1 is l2 True   gt  gt  gt  l1  l1   x    gt  gt  gt  l1 is l2 False   However in reality    gt  gt  gt  l2  l1  gt  gt  gt  l1     x    gt  gt  gt  l1 is l2 True   This is because Python lists implement   iadd     to make a    augmented assignment short-circuit and call list extend   instead   It s a bit of a strange wart this  it usually does what you meant  but for confusing reasons    In general  if you re appending extended an existing list  and you want to keep the reference to the same list  instead of making a new one   it s best to be explicit and stick with the append   extend   methods

User · Answer

The difference is that concatenate will flatten the resulting list  whereas append will keep the levels intact   So for example with   myList       listA    1 2 3  listB     a   b   c     Using append  you end up with a list of lists    gt  gt  myList append listA   gt  gt  myList append listB   gt  gt  myList   1 2 3    a  b   c      Using concatenate instead  you end up with a flat list    gt  gt  myList    listA   listB  gt  gt  myList  1 2 3  a   b   c

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gt  gt  gt  a     gt  gt  gt  a append  1 2    gt  gt  gt  a   1  2    gt  gt  gt  a     gt  gt  gt  a   1 2   gt  gt  gt  a  1  2    See that append adds a single element to the list  which may be anything       joins the lists

User · Answer

The rebinding behaviour mentioned in other answers does matter in certain circumstances    gt  gt  gt  a            gt  gt  gt  a 0  append 1   gt  gt  gt  a   1        gt  gt  gt  a 1      1  Traceback  most recent call last     File   lt interactive input gt    line 1  in  lt module gt  TypeError   tuple  object does not support item assignment   That s because augmented assignment always rebinds  even if the object was mutated in-place  The rebinding here happens to be a 1     mutated list   which doesn t work for tuples

User · Answer

For your case the only difference is performance  append is twice as fast   Python 3 0  r30 67507  Dec  3 2008  20 14 27   MSC v 1500 32 bit  Intel   on win32 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import timeit  gt  gt  gt  timeit Timer  s append  something      s        timeit   0 20177424499999999  gt  gt  gt  timeit Timer  s      something      s        timeit   0 41192320500000079  Python 2 5 1  r251 54863  Apr 18 2007  08 51 08   MSC v 1310 32 bit  Intel   on win32 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  import timeit  gt  gt  gt  timeit Timer  s append  something      s        timeit   0 23079359499999999  gt  gt  gt  timeit Timer  s      something      s        timeit   0 44208112500000141   In general case append will add one item to the list  while    will copy all elements of right-hand-side list into the left-hand-side list   Update  perf analysis  Comparing bytecodes we can assume that append version wastes cycles in LOAD ATTR   CALL FUNCTION  and    version -- in BUILD LIST  Apparently BUILD LIST outweighs LOAD ATTR   CALL FUNCTION     gt  gt  gt  import dis  gt  gt  gt  dis dis compile  s       s append  spam          exec      1           0 BUILD LIST               0               3 STORE NAME               0  s                6 LOAD NAME                0  s                9 LOAD ATTR                1  append               12 LOAD CONST               0   spam                15 CALL FUNCTION            1              18 POP TOP              19 LOAD CONST               1  None               22 RETURN VALUE  gt  gt  gt  dis dis compile  s       s      spam          exec      1           0 BUILD LIST               0               3 STORE NAME               0  s                6 LOAD NAME                0  s                9 LOAD CONST               0   spam                12 BUILD LIST               1              15 INPLACE ADD              16 STORE NAME               0  s               19 LOAD CONST               1  None               22 RETURN VALUE   We can improve performance even more by removing LOAD ATTR overhead    gt  gt  gt  timeit Timer  a  something      s       a   s append   timeit   0 15924410999923566

User · Answer

does not mutate the list   append   mutates the old list

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