python list by value not by reference

Question

Let s take an example  a   help    copyright    credits    license   b a b append  XYZ   b   help    copyright    credits    license    XYZ   a   help    copyright    credits    license    XYZ     I wanted to append value in list  b  but the value of list  a  have also changed   I think I have little idea why its like this  python passes lists by reference    My question is  how can I pass it by value so that appending  b  does t change values in  a

User · Answer

I found that we can use extend   to implement the function of copy    a   help    copyright    credits    license   b      b extend a  b append  XYZ

User · Answer

To copy a list you can use list a  or a     In both cases a new object is created  These two methods  however  have limitations with collections of mutable objects as inner objects keep their references intact    gt  gt  gt  a     1 2   3   4     gt  gt  gt  b   a     gt  gt  gt  c   list a    gt  gt  gt  c 0  append 9    gt  gt  gt  a   1  2  9    3    4    gt  gt  gt  c   1  2  9    3    4    gt  gt  gt  b   1  2  9    3    4    gt  gt  gt     If you want a full copy of your objects you need copy deepcopy   gt  gt  gt  from copy import deepcopy  gt  gt  gt  a     1 2   3   4     gt  gt  gt  b   a     gt  gt  gt  c   deepcopy a    gt  gt  gt  c 0  append 9    gt  gt  gt  a   1  2    3    4    gt  gt  gt  b   1  2    3    4    gt  gt  gt  c   1  2  9    3    4    gt  gt  gt

User · Answer

As answered in the official Python FAQ   b   a

User · Answer

When you do b   a you simply create another pointer to the same memory of a  that s why when you append to b   a changes too   You need to create copy of a and that s done like this   b   a

User · Answer

As mentioned by phihag in his answer   b   a      will work for your case since slicing a list creates a new memory id of the list  meaning you are no longer referencing the same object in your memory and the changes you make to one will not be reflected in the other    However  there is a slight problem   If your list is multidimensional  as in lists within lists  simply slicing will not solve this problem   Changes made in the higher dimensions  i e  the lists within the original list  will be shared between the two   Do not fret  there is a solution   The module copy has a nifty copying technique that takes care of this issue   from copy import deepcopy  b   deepcopy a    will copy a list with a new memory id no matter how many levels of lists it contains

User · Answer

b   list a   See http   henry precheur org python copy list

User · Answer

To create a copy of a list do this   b   a

User · Answer

If you want to copy a one-dimensional list  use  b   a      However  if a is a 2-dimensional list  this is not going to work for you  That is  any changes in a will also be reflected in b  In that case  use  b     a x  y  for y in range len a 0     for x in range len a

User · Answer

I would recommend the following solution   b      b      a   This will copy all the elements from a to b  The copy will be value copy  not reference copy

User · Answer

Also  you can do   b   list a    This will work for any sequence  even those that don t support indexers and slices

User · Answer

In terms of performance my favorite answer would be   b extend a    Check how the related alternatives compare with each other in terms of performance   In  1   import timeit  In  2   timeit timeit  b extend a    setup  b    a range 0 10    number 100000000  Out 2   9 623248100280762  In  3   timeit timeit  b   a      setup  b    a range 0 10    number 100000000  Out 3   10 84756088256836  In  4   timeit timeit  b   list a    setup  b    a range 0 10    number 100000000  Out 4   21 46313500404358  In  5   timeit timeit  b    elem for elem in a    setup  b    a range 0 10    number 100000000  Out 5   66 99795293807983  In  6   timeit timeit  for elem in a  b append elem    setup  b    a range 0 10    number 100000000  Out 6   67 9775960445404  In  7   timeit timeit  b   deepcopy a    setup  from copy import deepcopy  b    a range 0 10    number 100000000  Out 7   1216 1108016967773

[python] python list by value not by reference

Examples related to python

Examples related to list

Examples related to reference