Appending a list into a list of lists in python changes all sublists

Question

I am trying to create and append a list and index in a list. Appending any list element is being automatically appended to all the lists available in this list First of all I have a list as following

sygma_list [[]] * 3

and I have another lists having the form

mts_columns1 = [[1,2,3], [4,5,6], [6,7,8]]
mts_columns2 = [[1,2,3], [4,5,6], [6,7,8]]

When looping over the sygma_list I have like so:

 for i in range(0, 3):
       sygma_list[i].append(mts_column[i])

the results of sygma_list are being quite shocking, as append() is behaving on each element of the list instead of obtaining a final result of

sygma_list = [[[1,2,3], [1,2,3]],
              [[4,5,6],[4,5,6]],
              [[6,7,8],[6,7,8]]]

Answer 1

The biggest catch in your code is this: When you do this:

sygma_list = [[]] * 3

you create a size-3 array of references on the same list: not that you generally want and certainly not here

Do this instead:

sygma_list = [list() for x in range(3)]

That will create 3 distinct lists.

(this construct is OK for immutable objects like [0]*3 or["foo"]*4)

Let's consider this fixed code:

sygma_list = [list() for _ in range(3)]

mts_column = [[1,2,3], [4,5,6], [6,7,8]]

for i in range(0, 3):
    sygma_list[i].append(mts_column[i])

print(sygma_list)

yields:

[[[1, 2, 3]], [[4, 5, 6]], [[6, 7, 8]]]

BTW: Not sure if you want to append or extend your list (flatten it or not)

sygma_list[i].extend(mts_column[i])

would make a list of lists instead of making a list of lists of lists