Python issue with for loop and append

Question

I'm having trouble understanding the output of a piece of python code.

mani=[]
nima=[]
for i in range(3)
    nima.append(i)
    mani.append(nima)

print(mani)

The output is

[[0,1,2], [0,1,2], [0,1,2]]

I can't for the life of me understand why it is not

[[0], [0,1], [0,1,2]]

Any help much appreciated.

You appended `nima` (not a *copy* of `nima`) to `mani` 3 times, so it appears 3 times. — Scott Hunter, Jun 20 '20 at 14:32
Use https://docs.python.org/3/library/copy.html#module-copy to avoid this kind of behavior. You are using pointers rather than copies, as some have suggested, — Tomás Denis Reyes Sánchez, Jun 20 '20 at 14:39

score 6 · Accepted Answer · answered Jun 20 '20 at 14:32

6

It's because when you append nima into mani, it isn't a copy of nima, but a reference to nima.

So as nima changes, the reference at each location in mani, just points to the changed nima.

Since nima ends up as [0, 1, 2], then each reference appended into mani, just refers to the same object.

answered Jun 20 '20 at 14:32

PETER STACEY

2

Because a List is mutable it can be changed after it is created. :) – EdKenbers Jun 20 '20 at 14:35
Aha, and there it is, the hole in my understanding. I mainly used matlab in the past, and if i remember correctly it worked with the copy, as oppose to reference in such a situation (i think anyway!). Many thanks. – r3za Jun 20 '20 at 15:08

score 5 · Answer 2 · answered Jun 20 '20 at 14:43

5

Just to complete as some have suggested, you should use the copy module. Your code would look like:

import copy

mani=[]
nima=[]
for i in range(3):
    nima.append(i)
    mani.append(copy.copy(nima))

print(mani)

Output:

[[0], [0, 1], [0, 1, 2]]

answered Jun 20 '20 at 14:43

Is there a reason to use `copy.copy()` instead of `list.copy()`? i.e. `mani.append(nima.copy())` – wjandrea Jun 21 '20 at 02:35

score 1 · Answer 3 · answered Jun 20 '20 at 15:19

1

List are mutable (mutable sequences can be changed after they are created), you can see that you are operating on the same object using id function:

for i in mani:
    print(id(i))

answered Jun 20 '20 at 15:19

dejanualex

3 Answers3