For loop, list append

Question

I have a simple code

a_list=[1,2,3,4,5]
a2_list=[]
for x in a_list:
    a2_list.append(x*2)

and I get a2_list=[2,4,6,8,10]

If I write code like

a_list=[1,2,3,4,5]
a2_list=[]
for x in a_list:
    a2_list.append(x*2)
    print a2_list

I get

[2]
[2,4]
[2,4,6]
[2,4,6,8]
[2,4,6,8,10]

I want to make a list of lists to record each step

a_list=[1,2,3,4,5]
a2_list=[]
b_list=[]
for x in a_list:
    a2_list.append(x*2)
    b_list.append(a2_list)

I would like to get b_list = [[2],[2,4],[2,4,6],[2,4,6,8],[2,4,6,8,10]]

but I get b_list=[[2,4,6,8,10],[2,4,6,8,10],[2,4,6,8,10],[2,4,6,8,10],[2,4,6,8,10]]

It seems like a very easy problem, but I can't figure what I am doing wrong

first thing (could be just a copy-paste problem) you are missing a 'd' in append for b_list. — Jay T., Feb 24 '16 at 19:47
Possible duplicate of [How to clone or copy a list in Python?](http://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list-in-python) — jonrsharpe, Feb 24 '16 at 19:48

score 2 · Answer 1 · answered Feb 24 '16 at 19:55

2

That works

a_list=[1,2,3,4,5]
a2_list=[]
b_list=[]
for x in a_list:
    a2_list.append(x*2)
    b = a2_list[:]
    b_list.append(b)

answered Feb 24 '16 at 19:55

AK9309

score 0 · Answer 2 · edited May 23 '17 at 12:23

0

This is because list is mutable object. You should learn about it. The possible solution is to copy the list as others suggested. e.g.

a_list = [1,2,3,4,5]
a_list = list(a_list)
or
a_list = a_list[:]

There are other methods too.

edited May 23 '17 at 12:23

Community

answered Feb 25 '16 at 06:35

2 Answers2