Python list change index value of one after duplication

Question

As a hypothetical example:

>>> a = [1, 2, 3]
>>> b = a
>>> b[0] = 4
>>> b
[4, 2, 3]
>>> a
[4, 2, 3]

I know that this occurs because the references to both a and b are identical, and therefore point to the same bytes of memory. A change in the bytes referenced by b will be reflected in a, since they share the same memory.

What is the best way to work around this, so that a remains the original value, and after b is made from a, changes to b will not go to a, also.

Specific code:

outputs = []
a = [[0,2,5],[4,2,0],[6,0,0]]
for i in range(3):
  for j in range(3):
    if not a[i][j]:
      b = a
      b[i][j] = 1
      outputs.append(b)

This returns:

outputs = [[[1,2,5],[4,2,1],[6,1,1]],
           [[1,2,5],[4,2,1],[6,1,1]],
           [[1,2,5],[4,2,1],[6,1,1]]]

http://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list-in-python — Paul Rooney, Jun 29 '15 at 23:45
flag as a duplicate if you believe it is one, @Paul you can vote to close. — Ryan Haining, Jun 29 '15 at 23:53
http://stackoverflow.com/questions/1601269/how-to-make-a-completely-unshared-copy-of-a-complicated-list-deep-copy-is-not — Lafexlos, Jun 29 '15 at 23:59

user3636636 · Answer 1 · 2015-06-30T00:13:00.900

1

You can copy the original list by slicing it:

a = [1, 2, 3]
b = a[:]

Or using the built in list list() funtion:

b = list(a)

You could also use list comprehension in place of your nested for loops:

b = [[i if i else 1 for i in j] for j in a]

edited Jun 30 '15 at 00:13

answered Jun 29 '15 at 23:44

user3636636

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I have updated, the original answers were addressing the hypothetical situation you originally posted – user3636636 Jun 30 '15 at 00:20

score 0 · Answer 2 · answered Jun 29 '15 at 23:45

0

This :

new_list = list(old_list)

And so this will happen -

>>> a = [1,2,3]
>>> b = list(a)
>>> b.append(8)
>>> b
[1, 2, 3, 8]
>>> a
[1, 2, 3]

answered Jun 29 '15 at 23:45

Utsav T

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What about nested lists? This technique makes a shallow copy. – bcdan Jun 30 '15 at 00:14
For nested lists - http://stackoverflow.com/questions/7845152/deep-copy-nested-list-without-using-deepcopy-function – Utsav T Jun 30 '15 at 00:17

Python list change index value of one after duplication

2 Answers2