How to append values to list?

Question

I want to append my list with values for each loop iteration:

        for i in range (4,10):
        a_list = [1,2,3]
        a_list = a_list.append(i)

The wanted output would be [1,2,3,4,5,6,7,8,9]. But I get None. Also printing type(a_list) after using .append() gives me <class'NoneType'>. What is the problem here ?

`append` doesn't return a value, just write `a_list.append(i)` — ShadowMitia, May 12 '22 at 13:56
Does this answer your question? [Why does append() always return None in Python?](https://stackoverflow.com/questions/16641119/why-does-append-always-return-none-in-python) — mkrieger1, May 12 '22 at 14:00
Someone put a minus. Why make such hasty conclusions...?) Posted my own and the author's options. — inquirer, May 12 '22 at 14:09

score 1 · Answer 1 · answered May 12 '22 at 13:57

1

firstly, you have to mention a_list before for. instead, you will get [1, 2, 3, 9]. secondly, you give the a_list a value of a_list.append() function.

a_list = [1, 2, 3]
for i in range(4, 10):
    a_list.append(i)

answered May 12 '22 at 13:57

make-me-fly

31
4

racey chan · Accepted Answer · 2022-05-12T14:01:36.723

it is mainly because of the fact that list.append method does not return anything, it appends the given value to the list in-place.

we can confirm this by the simple example below

a = list()
b = a.append(5)
>> print(b)
None
>> print(a)
[5]

As matter of fact,there is a simpler and more performant way to achieve this

>> a_list = [1,2,3]
>> a_list += list(range(4, 10))
>> print(a_list)
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Here, we first shape a new list from a iterator range(4, 10), then we concate the two list together to get the list we want by adding the new list to the original list a_list.

by doing this, we can avoid the overhead caused by repeatedly call the list.append method in a for loop

inquirer · Answer 3 · 2022-05-12T14:11:35.687

a_list = []
for i in range (1,10):
    a_list.append(i)

print(a_list)

Output

[1, 2, 3, 4, 5, 6, 7, 8, 9]

You have the wrong indentation on the loop.You need to add i at each iteration. Also, to get the list you want, you need to add 1, then 2, then 3, and so on. i this is what needs to be added. Put print(i) and print each iteration.

a_list = [1,2,3]
for i in range (4,10):
    a_list.append(i)

print(a_list)

If you use your option, it will be correct to declare an array once. And then only add values. See below.

[1, 2, 3, 4, 5, 6, 7, 8, 9]

How to append values to list?

3 Answers3