How can I get the concatenation of two lists in Python without modifying either one

Question

In Python  the only way I can find to concatenate two lists is list extend  which modifies the first list  Is there any concatenation function that returns its result without modifying its arguments

User · Accepted Answer

Yes  list1   list2  This gives a new list that is the concatenation of list1 and list2

User · Answer

You can also use sum  if you give it a start argument    gt  gt  gt  list1  list2  list3    1 2 3     a   b   c     7 8 9   gt  gt  gt  all lists   sum  list1  list2  list3        gt  gt  gt  all lists  1  2  3   a    b    c   7  8  9    This works in general for anything that has the   operator    gt  gt  gt  sum   1 2    1              1  2  1    gt  gt  gt  sum  Counter  123    Counter  234    Counter  345     Counter    Counter   1  1   2  2   3  3   4  2   5  1     gt  gt  gt  sum  True  True  False   False  2   With the notable exception of strings    gt  gt  gt  sum   123    345    567        Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  sum   can t sum strings  use    join seq  instead

User · Answer

concatenated list   list 1   list 2

User · Answer

The simplest method is just to use the   operator  which returns the concatenation of the lists   concat   first list   second list  One disadvantage of this method is that twice the memory is now being used   For very large lists  depending on how you re going to use it once it s created  itertools chain might be your best bet    gt  gt  gt  import itertools  gt  gt  gt  a    1  2  3   gt  gt  gt  b    4  5  6   gt  gt  gt  c   itertools chain a  b    This creates a generator for the items in the combined list  which has the advantage that no new list needs to be created  but you can still use c as though it were the concatenation of the two lists    gt  gt  gt  for i in c          print i 1 2 3 4 5 6   If your lists are large and efficiency is a concern then this and other methods from the itertools module are very handy to know   Note that this example uses up the items in c  so you d need to reinitialise it before you can reuse it  Of course you can just use list c  to create the full list  but that will create a new list in memory

User · Answer

you could always create a new list which is a result of adding two lists    gt  gt  gt  k    1 2 3     4 7 9   gt  gt  gt  k  1  2  3  4  7  9    Lists are mutable sequences so I guess it makes sense to modify the original lists by extend or append

User · Answer

And if you have more than two lists to concatenate   import operator from functools import reduce    For Python 3 list1  list2  list3    1 2 3     a   b   c     7 8 9  reduce operator add   list1  list2  list3      or with an existing list all lists    list1  list2  list3  reduce operator add  all lists    It doesn t actually save you any time  intermediate lists are still created  but nice if you have a variable number of lists to flatten  e g    args

User · Answer

Just to let you know   When you write list1   list2  you are calling the   add   method of list1  which returns a new list  in this way you can also deal with myobject   list1 by adding the   add   method to your personal class

[python] How can I get the concatenation of two lists in Python without modifying either one?

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