How to deep copy a list

Question

I have some problem with a List copy     So After I got E0 from  get edge   I make a copy of E0 by calling  E0 copy   list E0    Here I guess E0 copy is a deep copy of E0  and I pass E0 copy into  karger E    But in the main function  Why does the result of  print E0 1 10   before the for loop is not the same with that after the for loop    Below is my code   def get graph        f open  kargerMinCut txt       G        for line in f          ints    int x  for x in line split            G ints 0   ints 1 len ints       return G  def get edge G       E        for i in range 1 201           for v in G i               if v gt i                  E append  i v       print id E      return E  def karger E       import random     count 200      while 1          if count    2              break         edge   random randint 0 len E -1          v0 E edge  0          v1 E edge  1                             E pop edge          if v0    v1              count -  1             i 0             while 1                  if i    len E                       break                 if E i  0     v1                      E i  0    v0                 if E i  1     v1                      E i  1    v0                 if E i  0     E i  1                       E pop i                      i- 1                 i  1      mincut len E      return mincut   if   name       main         import copy     G   get graph       results        E0   get edge G      print E0 1 10                   this result is not equal to print2     for k in range 1 5           E0 copy list E0             I guess here E0 coypy is a deep copy of E0         results append karger E0 copy           print  the result is  d   min results      print E0 1 10                   this is print2

User · Answer

If you are not allowed to directly import modules you can define your own deepcopy function as -

def copyList(L):
if type(L[0]) != list:
    return [i for i in L]
else:
    return [copyList(L[i]) for i in range(len(L))]

It's working can be seen easily as -

>>> x = [[1,2,3],[3,4]]
>>> z = copyList(x)
>>> x
[[1, 2, 3], [3, 4]]
>>> z
[[1, 2, 3], [3, 4]]
>>> id(x)
2095053718720
>>> id(z)
2095053718528
>>> id(x[0])
2095058990144
>>> id(z[0])
2095058992192
>>>

User · Answer

If the contents of the list are primitive data types  you can use a comprehension  new list    i for i in old list    You can nest it for multidimensional lists like   new grid     i for i in row  for row in grid

User · Answer

I believe a lot of programmers have run into one or two interview problems where they are asked to deep copy a linked list  however this problem is harder than it sounds   in python  there is a module called  copy  with two useful functions  import copy copy copy   copy deepcopy     copy   is a shallow copy function  if the given argument is a compound data structure  for instance a list  then python will create another object of the same type  in this case  a new list  but for everything inside old list  only their reference is copied    think of it like newList    elem for elem in oldlist    Intuitively  we could assume that deepcopy   would follow the same paradigm  and the only difference is that for each elem we will recursively call deepcopy   just like the answer of mbcoder   but this is wrong   deepcopy   actually preserve the graphical structure of the original compound data   a    1 2  b    a a    there s only 1 object a c   deepcopy b     check the result c 0  is a   return False  a new object a  is created c 0  is c 1    return True  c is  a  a   not  a  a      this is the tricky part  during the process of deepcopy   a hashtable dictionary in python  is used to map    old object ref onto new object ref   this prevent unnecessary duplicates and thus preserve the structure of the copied compound data  official doc

User · Answer

just a recursive deep copy function   def deepcopy A       rt          for elem in A          if isinstance elem list               rt append deepcopy elem           else              rt append elem      return rt   Edit  As Cfreak mentioned  this is already implemented in copy module

User · Answer

This is more pythonic  my list    0  1  2  3  4  5     some list my list copy   list my list     my list copy and my list does not share reference now    NOTE  This is not safe with a list of referenced objects

User · Answer

Regarding the list as a tree  the deep copy in python can be most compactly written as  def deep copy x       if not isinstance x  list   return x     else  return map deep copy  x

User · Answer

Here s an example of how to deep copy a 2D list    b    x    for x in a

User · Answer

E0 copy is not a deep copy  You don t make a deep copy using list    Both list      and testList    are shallow copies    You use copy deepcopy      for deep copying a list   deepcopy x  memo None   nil         Deep copy operation on arbitrary Python objects    See the following snippet -    gt  gt  gt  a     1  2  3    4  5  6    gt  gt  gt  b   list a   gt  gt  gt  a   1  2  3    4  5  6    gt  gt  gt  b   1  2  3    4  5  6    gt  gt  gt  a 0  1    10  gt  gt  gt  a   1  10  3    4  5  6    gt  gt  gt  b     b changes too - gt  Not a deepcopy    1  10  3    4  5  6     Now see the deepcopy operation   gt  gt  gt  import copy  gt  gt  gt  b   copy deepcopy a   gt  gt  gt  a   1  10  3    4  5  6    gt  gt  gt  b   1  10  3    4  5  6    gt  gt  gt  a 0  1    9  gt  gt  gt  a   1  9  3    4  5  6    gt  gt  gt  b      b doesn t change - gt  Deep Copy   1  10  3    4  5  6

User · Answer

If your list elements are immutable objects then you can use this  otherwise you have to use deepcopy from copy module    you can also use shortest way for deep copy a list like this    a    0 1 2 3 4 5 6 7 8 9 10  b   a     deep copying the list a and assigning it to b print id a  20983280 print id b  12967208  a 2    20 print a  0  1  20  3  4  5  6  7  8  9 10  print b  0  1  2  3  4  5  6  7  8  9 10

User · Answer

Sukrit Kalra No 1  list         copy copy   are all shallow copy  If an object is compound  they are all not suitable  You need to use copy deepcopy    No 2  b   a directly  a and b have the same reference  changing a is even as changing b    python Python 3 9 0  tags v3 9 0 9cf6752  Oct  5 2020  15 34 40   MSC v 1927 64 bit  AMD64   on win32 Type  quot help quot    quot copyright quot    quot credits quot  or  quot license quot  for more information   gt  gt  gt  a     1  2  3    4  5  6    gt  gt  gt  b   list a   gt  gt  gt  a   1  2  3    4  5  6    gt  gt  gt  b   1  2  3    4  5  6    gt  gt  gt  a 0    1  gt  gt  gt  a  1   4  5  6    gt  gt  gt  b   1  2  3    4  5  6    gt  gt  gt  exit      python Python 3 9 0  tags v3 9 0 9cf6752  Oct  5 2020  15 34 40   MSC v 1927 64 bit  AMD64   on win32 Type  quot help quot    quot copyright quot    quot credits quot  or  quot license quot  for more information   gt  gt  gt  a     1  2  3    4  5  6    gt  gt  gt  b   a  gt  gt  gt  a   1  2  3    4  5  6    gt  gt  gt  b   1  2  3    4  5  6    gt  gt  gt  a 0    1  gt  gt  gt  a  1   4  5  6    gt  gt  gt  b  1   4  5  6    gt  gt  gt  exit

[python] How to deep copy a list?

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