List changes unexpectedly after assignment How do I clone or copy it to prevent this

Question

While using new list   my list  any modifications to new list changes my list every time  Why is this  and how can I clone or copy the list to prevent it

User · Answer

Note that there are some cases where if you have defined your own custom class and you want to keep the attributes then you should use copy.copy() or copy.deepcopy() rather than the alternatives, for example in Python 3:

import copy

class MyList(list):
    pass

lst = MyList([1,2,3])

lst.name = 'custom list'

d = {
'original': lst,
'slicecopy' : lst[:],
'lstcopy' : lst.copy(),
'copycopy': copy.copy(lst),
'deepcopy': copy.deepcopy(lst)
}


for k,v in d.items():
    print('lst: {}'.format(k), end=', ')
    try:
        name = v.name
    except AttributeError:
        name = 'NA'
    print('name: {}'.format(name))

Outputs:

lst: original, name: custom list
lst: slicecopy, name: NA
lst: lstcopy, name: NA
lst: copycopy, name: custom list
lst: deepcopy, name: custom list

User · Answer

Remember that in Python when you do       list1     apples   bananas   pineapples       list2   list1   List2 isn t storing the actual list  but a reference to list1  So when you do anything to list1  list2 changes as well  use the copy module  not default  download on pip  to make an original copy of the list copy copy   for simple lists  copy deepcopy   for nested ones   This makes a copy that doesn t change with the first list

User · Answer

Felix already provided an excellent answer  but I thought I d do a speed comparison of the various methods    10 59 sec  105 9us itn  -  copy deepcopy old list  10 16 sec  101 6us itn  - pure python Copy   method copying classes with deepcopy 1 488 sec  14 88us itn  - pure python Copy   method not copying classes  only dicts lists tuples  0 325 sec  3 25us itn  - for item in old list  new list append item  0 217 sec  2 17us itn  -  i for i in old list   a list comprehension  0 186 sec  1 86us itn  - copy copy old list  0 075 sec  0 75us itn  - list old list  0 053 sec  0 53us itn  - new list       new list extend old list  0 039 sec  0 39us itn  - old list     list slicing    So the fastest is list slicing  But be aware that copy copy    list    and list list   unlike copy deepcopy   and the python version don t copy any lists  dictionaries and class instances in the list  so if the originals change  they will change in the copied list too and vice versa    Here s the script if anyone s interested or wants to raise any issues    from copy import deepcopy  class old class      def   init   self           self blah    blah   class new class object       def   init   self           self blah    blah   dignore    str  None  unicode  None  int  None  type None   None   def Copy obj  use deepcopy True       t   type obj       if t in  list  tuple           if t    tuple                Convert to a list if a tuple to                allow assigning to when copying             is tuple   True             obj   list obj          else                 Otherwise just do a quick slice copy             obj   obj                is tuple   False            Copy each item recursively         for x in xrange len obj                if type obj x   in dignore                  continue             obj x    Copy obj x   use deepcopy           if is tuple                 Convert back into a tuple again             obj   tuple obj       elif t    dict             Use the fast shallow dict copy   method and copy any            values which aren t immutable  like lists  dicts etc          obj   obj copy           for k in obj              if type obj k   in dignore                  continue             obj k    Copy obj k   use deepcopy       elif t in dignore             Numeric or string unicode             It s immutable  so ignore it          pass       elif use deepcopy           obj   deepcopy obj      return obj  if   name         main         import copy     from time import time      num times   100000     L    None   blah   1  543 4532              foo      bar       blah    blah             old class    new class         t   time       for i in xrange num times           Copy L      print  Custom Copy    time  -t      t   time       for i in xrange num times           Copy L  use deepcopy False      print  Custom Copy Only Copying Lists Tuples Dicts  no classes     time  -t      t   time       for i in xrange num times           copy copy L      print  copy copy    time  -t      t   time       for i in xrange num times           copy deepcopy L      print  copy deepcopy    time  -t      t   time       for i in xrange num times           L        print  list slicing        time  -t      t   time       for i in xrange num times           list L      print  list L     time  -t      t   time       for i in xrange num times            i for i in L      print  list expression L     time  -t      t   time       for i in xrange num times           a              a extend L      print  list extend    time  -t      t   time       for i in xrange num times           a              for y in L              a append y      print  list append    time  -t      t   time       for i in xrange num times           a              a extend i for i in L      print  generator expression extend    time  -t

User · Answer

Python s idiom for doing this is newList   oldList

User · Answer

I wanted to post something a bit different then some of the other answers  Even though this is most likely not the most understandable  or fastest option  it provides a bit of an inside view of how deep copy works  as well as being another alternative option for deep copying  It doesn t really matter if my function has bugs  since the point of this is to show a way to copy objects like the question answers  but also to use this as a point to explain how deepcopy works at its core   At the core of any deep copy function is way to make a shallow copy  How  Simple  Any deep copy function only duplicates the containers of immutable objects  When you deepcopy a nested list  you are only duplicating the outer lists  not the mutable objects inside of the lists  You are only duplicating the containers  The same works for classes  too  When you deepcopy a class  you deepcopy all of its mutable attributes  So  how  How come you only have to copy the containers  like lists  dicts  tuples  iters  classes  and class instances   It s simple  A mutable object can t really be duplicated  It can never be changed  so it is only a single value  That means you never have to duplicate strings  numbers  bools  or any of those  But how would you duplicate the containers  Simple  You make just initialize a new container with all of the values  Deepcopy relies on recursion  It duplicates all the containers  even ones with containers inside of them  until no containers are left  A container is an immutable object   Once you know that  completely duplicating an object without any references is pretty easy  Here s a function for deepcopying basic data-types  wouldn t work for custom classes but you could always add that   def deepcopy x     immutables    str  int  bool  float    mutables    list  dict  tuple    if isinstance x  immutables       return x   elif isinstance x  mutables       if isinstance x  tuple         return tuple deepcopy list x        elif isinstance x  list         return  deepcopy y  for y in x      elif isinstance x  dict         values    deepcopy y  for y in list x values           keys   list x keys          return dict zip keys  values     Python s own built-in deepcopy is based around that example  The only difference is it supports other types  and also supports user-classes by duplicating the attributes into a new duplicate class  and also blocks infinite-recursion with a reference to an object it s already seen using a memo list or dictionary  And that s really it for making deep copies  At its core  making a deep copy is just making shallow copies  I hope this answer adds something to the question   EXAMPLES  Say you have this list   1  2  3   The immutable numbers cannot be duplicated  but the other layer can  You can duplicate it using a list comprehension   x for x in  1  2  3   Now  imagine you have this list    1  2    3  4    5  6    This time  you want to make a function  which uses recursion to deep copy all layers of the list  Instead of the previous list comprehension    x for x in  list    It uses a new one for lists    deepcopy list x  for x in  list    And deepcopy list looks like this   def deepcopy list x     if isinstance x   str  bool  float  int        return x   else      return  deepcopy list y  for y in x    Then now you have a function which can deepcopy any list of strs  bools  floast  ints and even lists to infinitely many layers using recursion  And there you have it  deepcopying   TLDR  Deepcopy uses recursion to duplicate objects  and merely returns the same immutable objects as before  as immutable objects cannot be duplicated  However  it deepcopies the most inner layers of mutable objects until it reaches the outermost mutable layer of an object

User · Answer

What are the options to clone or copy a list in Python    In Python 3  a shallow copy can be made with   a copy   a list copy     In Python 2 and 3  you can get a shallow copy with a full slice of the original   a copy   a list      Explanation  There are two semantic ways to copy a list  A shallow copy creates a new list of the same objects  a deep copy creates a new list containing new equivalent objects   Shallow list copy  A shallow copy only copies the list itself  which is a container of references to the objects in the list  If the objects contained themselves are mutable and one is changed  the change will be reflected in both lists    There are different ways to do this in Python 2 and 3  The Python 2 ways will also work in Python 3   Python 2  In Python 2  the idiomatic way of making a shallow copy of a list is with a complete slice of the original   a copy   a list      You can also accomplish the same thing by passing the list through the list constructor    a copy   list a list    but using the constructor is less efficient    gt  gt  gt  timeit  gt  gt  gt  l   range 20   gt  gt  gt  min timeit repeat lambda  l      0 30504298210144043  gt  gt  gt  min timeit repeat lambda  list l    0 40698814392089844   Python 3  In Python 3  lists get the list copy method   a copy   a list copy     In Python 3 5    gt  gt  gt  import timeit  gt  gt  gt  l   list range 20    gt  gt  gt  min timeit repeat lambda  l      0 38448613602668047  gt  gt  gt  min timeit repeat lambda  list l    0 6309100328944623  gt  gt  gt  min timeit repeat lambda  l copy     0 38122922903858125   Making another pointer does not make a copy     Using new list   my list then modifies new list every time my list changes  Why is this    my list is just a name that points to the actual list in memory  When you say new list   my list you re not making a copy  you re just adding another name that points at that original list in memory  We can have similar issues when we make copies of lists     gt  gt  gt  l                 gt  gt  gt  l copy   l     gt  gt  gt  l copy               gt  gt  gt  l copy 0  append  foo    gt  gt  gt  l copy    foo             gt  gt  gt  l    foo              The list is just an array of pointers to the contents  so a shallow copy just copies the pointers  and so you have two different lists  but they have the same contents  To make copies of the contents  you need a deep copy   Deep copies  To make a deep copy of a list  in Python 2 or 3  use deepcopy in the copy module   import copy a deep copy   copy deepcopy a list    To demonstrate how this allows us to make new sub-lists    gt  gt  gt  import copy  gt  gt  gt  l    foo             gt  gt  gt  l deep copy   copy deepcopy l   gt  gt  gt  l deep copy 0  pop    foo   gt  gt  gt  l deep copy               gt  gt  gt  l    foo              And so we see that the deep copied list is an entirely different list from the original  You could roll your own function - but don t  You re likely to create bugs you otherwise wouldn t have by using the standard library s deepcopy function   Don t use eval  You may see this used as a way to deepcopy  but don t do it   problematic deep copy   eval repr a list      It s dangerous  particularly if you re evaluating something from a source you don t trust  It s not reliable  if a subelement you re copying doesn t have a representation that can be eval d to reproduce an equivalent element  It s also less performant     In 64 bit Python 2 7    gt  gt  gt  import timeit  gt  gt  gt  import copy  gt  gt  gt  l   range 10   gt  gt  gt  min timeit repeat lambda  copy deepcopy l    27 55826997756958  gt  gt  gt  min timeit repeat lambda  eval repr l     29 04534101486206   on 64 bit Python 3 5    gt  gt  gt  import timeit  gt  gt  gt  import copy  gt  gt  gt  l   list range 10    gt  gt  gt  min timeit repeat lambda  copy deepcopy l    16 84255409205798  gt  gt  gt  min timeit repeat lambda  eval repr l     34 813894678023644

User · Answer

This is because  the line new list   my list assigns a new reference to the variable my list which is new list This is similar to the C code given below   int my list      1 2 3 4   int  new list  new list   my list    You should use the copy module to create a new list by  import copy new list   copy deepcopy my list

User · Answer

A slight practical perspective to look into memory through id and gc     gt  gt  gt  b   a     hell    word    gt  gt  gt  c     hell    word     gt  gt  gt  id a   id b   id c   4424020872  4424020872  4423979272                            -----------   gt  gt  gt  id a 0    id b 0    id c 0    4424018328  4424018328  4424018328    all referring to same  hell                                       -----------------------   gt  gt  gt  id a 0  0    id b 0  0    id c 0  0    4422785208  4422785208  4422785208    all referring to same  h                                       -----------------------   gt  gt  gt  a 0      o   gt  gt  gt  a b c    hello    word      hello    word      hell    word       b changed too  gt  gt  gt  id a 0    id b 0    id c 0    4424018384  4424018384  4424018328    augmented assignment changed a 0  b 0                           -----------   gt  gt  gt  b   a     hell    word    gt  gt  gt  id a 0    id b 0    id c 0    4424018328  4424018328  4424018328    the same hell                                      -----------------------   gt  gt  gt  import gc  gt  gt  gt  gc get referrers a 0       hell    word      hell    word       one copy belong to a b  the another for c  gt  gt  gt  gc get referrers   hell       hell    word      hell    word      hell   None       hello   None

User · Answer

Python 3 6 Timings  Here are the timing results using Python 3 6 8  Keep in mind these times are relative to one another  not absolute   I stuck to only doing shallow copies  and also added some new methods that weren t possible in Python2  such as list copy    the Python3 slice equivalent  and two forms of list unpacking   new list    list and new list     list     METHOD                  TIME TAKEN b     a                 2 75180600000021 b   a   1               3 50215399999990 b   a                   3 78278899999986    Python2 winner  see above  b   a copy              4 20556500000020    Python3  slice equivalent   see above  b       b extend a      4 68069800000012 b   a 0 len a           6 84498999999959  b    a                 7 54031799999984 b   list a              7 75815899999997 b    i for i in a       18 4886440000000 b   copy copy a         18 8254879999999 b      for item in a    b append item         35 4729199999997   We can see the Python2 winner still does well  but doesn t edge out Python3 list copy   by much  especially considering the superior readability of the latter   The dark horse is the unpacking and repacking method  b     a    which is  25  faster than raw slicing  and more than twice as fast as the other unpacking method   b    a    b   a   1 also does surprisingly well   Note that these methods do not output equivalent results for any input other than lists  They all work for sliceable objects  a few work for any iterable  but only copy copy   works for more general Python objects     Here is the testing code for interested parties  Template from here    import timeit  COUNT   50000000 print  Array duplicating  Tests run   COUNT   times   setup    a    0 1 2 3 4 5 6 7 8 9   import copy   print  b   list a  t t   timeit timeit stmt  b   list a    setup setup  number COUNT   print  b   copy copy a  t   timeit timeit stmt  b   copy copy a    setup setup  number COUNT   print  b   a copy   t t   timeit timeit stmt  b   a copy     setup setup  number COUNT   print  b   a    t t   timeit timeit stmt  b   a      setup setup  number COUNT   print  b   a 0 len a   t t   timeit timeit stmt  b   a 0 len a     setup setup  number COUNT   print   b    a t t t   timeit timeit stmt   b    a   setup setup  number COUNT   print  b       b extend a  t   timeit timeit stmt  b       b extend a    setup setup  number COUNT   print  b       for item in a  b append item  t   timeit timeit stmt  b      nfor item in a   b append item    setup setup  number COUNT   print  b    i for i in a  t   timeit timeit stmt  b    i for i in a    setup setup  number COUNT   print  b     a  t t   timeit timeit stmt  b     a    setup setup  number COUNT   print  b   a   1 t t   timeit timeit stmt  b   a   1   setup setup  number COUNT

User · Answer

There are many answers already that tell you how to make a proper copy  but none of them say why your original  copy  failed    Python doesn t store values in variables  it binds names to objects  Your original assignment took the object referred to by my list and bound it to new list as well  No matter which name you use there is still only one list  so changes made when referring to it as my list will persist when referring to it as new list  Each of the other answers to this question give you different ways of creating a new object to bind to new list    Each element of a list acts like a name  in that each element binds non-exclusively to an object  A shallow copy creates a new list whose elements bind to the same objects as before   new list   list my list     or my list     but I prefer this syntax   is simply a shorter way of  new list    element for element in my list    To take your list copy one step further  copy each object that your list refers to  and bind those element copies to a new list    import copy     each element must have   copy   defined for this    new list    copy copy element  for element in my list    This is not yet a deep copy  because each element of a list may refer to other objects  just like the list is bound to its elements  To recursively copy every element in the list  and then each other object referred to by each element  and so on  perform a deep copy    import copy   each element must have   deepcopy   defined for this    new list   copy deepcopy my list    See the documentation for more information about corner cases in copying

User · Answer

There is a simple technique to handle this  Code  number  1 2 3 4 5 6   Original list another     another empty list for a in number   here I am declaring variable  a  as an item in the list  number      another append a   here we are adding the items of list  number  to list  another  print another   Output   gt  gt  gt   1 2 3 4 5 6   I hope this was useful for your query

User · Answer

Use thing      gt  gt  gt  a    1 2   gt  gt  gt  b   a     gt  gt  gt  a     3   gt  gt  gt  a  1  2  3   gt  gt  gt  b  1  2   gt  gt  gt

User · Answer

The method to use depends on the contents of the list being copied  If the list contains nested dicts than deepcopy is the only method that works  otherwise most of the methods listed in the answers  slice  loop  for   copy  extend  combine  or unpack  will work and execute in similar time  except for loop and deepcopy  which preformed the worst   Script from random import randint from time import time import copy  item count   100000  def copy type l1  list  l2  list     if l1    l2      return  shallow    return  deep   def run time start  end     run   end - start   return int run   1000000   def list combine data     l1    data for i in range item count     start   time     l2        l1   end   time     if type data     dict      l2 0   test   append 1    elif type data     list      l2 append 1    return   method    combine    copy type   copy type l1  l2               time   s   run time start  end    def list extend data     l1    data for i in range item count     start   time     l2        l2 extend l1    end   time     if type data     dict      l2 0   test   append 1    elif type data     list      l2 append 1    return   method    extend    copy type   copy type l1  l2               time   s   run time start  end    def list unpack data     l1    data for i in range item count     start   time     l2     l1    end   time     if type data     dict      l2 0   test   append 1    elif type data     list      l2 append 1    return   method    unpack    copy type   copy type l1  l2               time   s   run time start  end    def list deepcopy data     l1    data for i in range item count     start   time     l2   copy deepcopy l1    end   time     if type data     dict      l2 0   test   append 1    elif type data     list      l2 append 1    return   method    deepcopy    copy type   copy type l1  l2               time   s   run time start  end    def list copy data     l1    data for i in range item count     start   time     l2   list copy l1    end   time     if type data     dict      l2 0   test   append 1    elif type data     list      l2 append 1    return   method    copy    copy type   copy type l1  l2               time   s   run time start  end    def list slice data     l1    data for i in range item count     start   time     l2   l1      end   time     if type data     dict      l2 0   test   append 1    elif type data     list      l2 append 1    return   method    slice    copy type   copy type l1  l2               time   s   run time start  end    def list loop data     l1    data for i in range item count     start   time     l2        for i in range len l1        l2 append l1 i     end   time     if type data     dict      l2 0   test   append 1    elif type data     list      l2 append 1    return   method    loop    copy type   copy type l1  l2               time   s   run time start  end    def list list data     l1    data for i in range item count     start   time     l2   list l1    end   time     if type data     dict      l2 0   test   append 1    elif type data     list      l2 append 1    return   method    list      copy type   copy type l1  l2               time   s   run time start  end    if   name         main       list type      list dict      test    1  1                  list list     1  1      store        for data in list type      key   list data keys    0      store append  key   list unpack data key    list extend data key                     list combine data key    list deepcopy data key                     list copy data key    list slice data key                               list loop data key        print store   Results    quot list dict  quot         quot method quot    quot unpack quot    quot copy type quot    quot shallow quot    quot time   s quot   56149       quot method quot    quot extend quot    quot copy type quot    quot shallow quot    quot time   s quot   52991       quot method quot    quot combine quot    quot copy type quot    quot shallow quot    quot time   s quot   53726       quot method quot    quot deepcopy quot    quot copy type quot    quot deep quot    quot time   s quot   2702616       quot method quot    quot copy quot    quot copy type quot    quot shallow quot    quot time   s quot   52204       quot method quot    quot slice quot    quot copy type quot    quot shallow quot    quot time   s quot   52223       quot method quot    quot loop quot    quot copy type quot    quot shallow quot    quot time   s quot   836928       quot list list  quot         quot method quot    quot unpack quot    quot copy type quot    quot deep quot    quot time   s quot   52313       quot method quot    quot extend quot    quot copy type quot    quot deep quot    quot time   s quot   52550       quot method quot    quot combine quot    quot copy type quot    quot deep quot    quot time   s quot   53203       quot method quot    quot deepcopy quot    quot copy type quot    quot deep quot    quot time   s quot   2608560       quot method quot    quot copy quot    quot copy type quot    quot deep quot    quot time   s quot   53210       quot method quot    quot slice quot    quot copy type quot    quot deep quot    quot time   s quot   52937       quot method quot    quot loop quot    quot copy type quot    quot deep quot    quot time   s quot   834774

User · Answer

The deepcopy option is the only method that works for me   from copy import deepcopy  a         list range 1  3   for i in range 3        b   deepcopy a  b 0  1   3  print  Deep    print a  print b  print  -----------------------------   a         list range 1  3   for i in range 3        b   a 1 b 0  1   3  print   1    print a  print b  print  -----------------------------   a         list range 1  3   for i in range 3      b   a    b 0  1   3  print  Vector copy    print a  print b  print  -----------------------------   a         list range 1  3   for i in range 3       b   list a  b 0  1   3  print  List copy    print a  print b  print  -----------------------------   a         list range 1  3   for i in range 3       b   a copy   b 0  1   3  print   copy      print a  print b  print  -----------------------------   a         list range 1  3   for i in range 3       b   a b 0  1   3  print  Shallow    print a  print b  print  -----------------------------     leads to output of   Deep     1  2    1  2    1  2       1  2    3    1  2    -----------------------------  1     1  2    3    1  2       1  2    3    1  2    ----------------------------- Vector copy     1  2    3    1  2       1  2    3    1  2    ----------------------------- List copy     1  2    3    1  2       1  2    3    1  2    -----------------------------  copy       1  2    3    1  2       1  2    3    1  2    ----------------------------- Shallow     1  2    3    1  2       1  2    3    1  2    -----------------------------

User · Answer

All of the other contributors gave great answers  which work when you have a single dimension  leveled  list  however of the methods mentioned so far  only copy deepcopy   works to clone copy a list and not have it point to the nested list objects when you are working with multidimensional  nested lists  list of lists   While Felix Kling refers to it in his answer  there is a little bit more to the issue and possibly a workaround using built-ins that might prove a faster alternative to deepcopy  While new list   old list     copy copy old list   and for Py3k old list copy   work for single-leveled lists  they revert to pointing at the list objects nested within the old list and the new list  and changes to one of the list objects are perpetuated in the other  Edit  New information brought to light  As was pointed out by both Aaron Hall and PM 2Ring using eval   is not only a bad idea  it is also much slower than copy deepcopy    This means that for multidimensional lists  the only option is copy deepcopy    With that being said  it really isn t an option as the performance goes way south when you try to use it on a moderately sized multidimensional array   I tried to timeit using a 42x42 array  not unheard of or even that large for bioinformatics applications  and I gave up on waiting for a response and just started typing my edit to this post  It would seem that the only real option then is to initialize multiple lists and work on them independently  If anyone has any other suggestions  for how to handle multidimensional list copying  it would be appreciated   As others have stated  there  are significant performance issues using the copy module and copy deepcopy for multidimensional lists

User · Answer

A very simple approach independent of python version was missing in already given answers which you can use most of the time  at least I do    new list   my list   1        Solution 1 when you are not using nested lists   However  If my list contains other containers  for eg  nested lists  you must use deepcopy as others suggested in the answers above from the copy library  For example   import copy new list   copy deepcopy my list     Solution 2 when you are using nested lists    Bonus  If you don t want to copy elements use  aka shallow copy    new list   my list        Let s understand difference between Solution 1 and Solution  2   gt  gt  gt  a   range 5   gt  gt  gt  b   a 1  gt  gt  gt  a b   0  1  2  3  4    0  1  2  3  4    gt  gt  gt  a 2    55   gt  gt  gt  a b   0  1  55  3  4    0  1  2  3  4     As you can see Solution  1 worked perfectly when we were not using the nested lists  Let s check what will happen when we apply solution  1 to nested lists    gt  gt  gt  from copy import deepcopy  gt  gt  gt  a    range i i 4  for i in range 3    gt  gt  gt  a   0  1  2  3    1  2  3  4    2  3  4  5    gt  gt  gt  b   a 1  gt  gt  gt  c   deepcopy a   gt  gt  gt  for i in  a  b  c   print i      0  1  2  3    1  2  3  4    2  3  4  5     0  1  2  3    1  2  3  4    2  3  4  5     0  1  2  3    1  2  3  4    2  3  4  5    gt  gt  gt  a 2  append  99    gt  gt  gt  for i in  a  b  c   print i      0  1  2  3    1  2  3  4    2  3  4  5  99     0  1  2  3    1  2  3  4    2  3  4  5  99      Solution 1 didn t work in nested list   0  1  2  3    1  2  3  4    2  3  4  5          Solution  2 - DeepCopy worked in nested list

User · Answer

There is another way of copying a list that was not listed until now   adding an empty list   l2   l       I tested it with Python 3 8   l    1 2 3  l2   l      print l l2  l 0     a  print l l2   Not the best answer but it works

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With new list   my list  you don t actually have two lists  The assignment just copies the reference to the list  not the actual list  so both new list and my list refer to the same list after the assignment   To actually copy the list  you have various possibilities    You can use the builtin list copy   method  available since Python 3 3    new list   old list copy    You can slice it    new list   old list      Alex Martelli s opinion  at least back in 2007  about this is  that it is a weird syntax and it does not make sense to use it ever      In his opinion  the next one is more readable   You can use the built in list   function   new list   list old list   You can use generic copy copy     import copy new list   copy copy old list    This is a little slower than list   because it has to find out the datatype of old list first  If the list contains objects and you want to copy them as well  use generic copy deepcopy     import copy new list   copy deepcopy old list    Obviously the slowest and most memory-needing method  but sometimes unavoidable    Example   import copy  class Foo object       def   init   self  val            self val   val      def   repr   self           return  Foo   r    format self val   foo   Foo 1   a     foo   foo  b   a copy   c   a    d   list a  e   copy copy a  f   copy deepcopy a     edit orignal list and instance  a append  baz   foo val   5  print  original   r nlist copy     r nslice   r nlist     r ncopy   r ndeepcopy   r           a  b  c  d  e  f     Result   original    foo   Foo 5    baz   list copy      foo   Foo 5   slice    foo   Foo 5   list      foo   Foo 5   copy    foo   Foo 5   deepcopy    foo   Foo 1

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Let s start from the beginning and explore this question   So let s suppose you have two lists   list 1   01   98   list 2    01   98      And we have to copy both lists  now starting from the first list   So first let s try by setting the variable copy to our original list  list 1   copy list 1   Now if you are thinking copy copied the list 1  then you are wrong  The id function can show us if two variables can point to the same object  Let s try this   print id copy   print id list 1     The output is   4329485320 4329485320   Both variables are the exact same argument  Are you surprised   So as we know python doesn t store anything in a variable  Variables are just referencing to the object and object store the value  Here object is a list but we created two references to that same object by two different variable names  This means that both variables are pointing to the same object  just with different names   When you do copy list 1  it is actually doing     Here in the image list 1 and copy are two variable names but the object is same for both variable which is list  So if you try to modify copied list then it will modify the original list too because the list is only one there  you will modify that list no matter you do from the copied list or from the original list   copy 0   modify   print copy  print list 1    output     modify    98     modify    98     So it modified the original list    Now let s move onto a pythonic method for copying lists   copy 1 list 1      This method fixes the first issue we had   print id copy 1   print id list 1    4338792136 4338791432   So as we can see our both list having different id and it means that both variables are pointing to different objects  So what actually going on here is     Now let s try to modify the list and let s see if we still face the previous problem   copy 1 0   modify   print list 1  print copy 1    The output is     01    98     modify    98     As you can see  it only modified the copied list  That means it worked   Do you think we re done  No  Let s try to copy our nested list   copy 2 list 2      list 2 should reference to another object which is copy of list 2  Let s check   print id  list 2   id copy 2     We get the output   4330403592 4330403528   Now we can assume both lists are pointing different object  so now let s try to modify it and let s see it is giving what we want   copy 2 0  1   modify   print list 2 copy 2    This gives us the output      01    modify       01    modify      This may seem a little bit confusing  because the same method we previously used worked  Let s try to understand this   When you do   copy 2 list 2      You re only copying the outer list  not the inside list  We can use the id function once again to check this   print id copy 2 0    print id list 2 0      The output is   4329485832 4329485832   When we do copy 2 list 2     this happens     It creates the copy of list but only outer list copy  not the nested list copy  nested list is same for both variable  so if you try to modify the nested list then it will modify the original list too as the nested list object is same for both lists   What is the solution  The solution is the deepcopy function   from copy import deepcopy deep deepcopy list 2    Let s check this   print id  list 2   id deep    4322146056 4322148040   Both outer lists have different IDs  let s try this on the inner nested lists   print id deep 0    print id list 2 0      The output is   4322145992 4322145800   As you can see both IDs are different  meaning we can assume that both nested lists are pointing different object now   This means when you do deep deepcopy list 2  what actually happens     Both nested lists are pointing different object and they have separate copy of nested list now   Now let s try to modify the nested list and see if it solved the previous issue or not   deep 0  1   modify  print list 2 deep    It outputs      01    98       01    modify      As you can see  it didn t modify the original nested list  it only modified the copied list

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It surprises me that this hasn t been mentioned yet  so for the sake of completeness     You can perform list unpacking with the  splat operator      which will also copy elements of your list   old list    1  2  3   new list     old list   new list append 4  old list     1  2  3  new list     1  2  3  4    The obvious downside to this method is that it is only available in Python 3 5    Timing wise though  this appears to perform better than other common methods   x    random random   for   in range 1000     timeit a   list x   timeit a   x copy    timeit a   x      timeit a     x      2 47   s    38 1 ns per loop  mean    std  dev  of 7 runs  100000 loops each     2 47   s    54 6 ns per loop  mean    std  dev  of 7 runs  100000 loops each     2 39   s    58 2 ns per loop  mean    std  dev  of 7 runs  100000 loops each      2 22   s    43 2 ns per loop  mean    std  dev  of 7 runs  100000 loops each

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I ve been told that Python 3 3  adds list copy   method  which should be as fast as slicing   newlist   old list copy

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new list   my list      new list   my list Try to understand this  Let s say that my list is in the heap memory at location X i e  my list is pointing to the X  Now by assigning new list   my list you re Letting new list pointing to the X  This is known as shallow Copy                                                                 Now if you assign new list   my list    You re simply copying each object of my list to new list  This is known as Deep copy   The Other way you can do this are     new list   list old list  import copy new list   copy deepcopy old list

[python] List changes unexpectedly after assignment. How do I clone or copy it to prevent this?

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