Question

I know that a list L can be copied by doing L[:]. But I face an issue that I do not understand why.

src = [1,2,3]
dest = [[5,6,7]]
dest.append(src[:].append(4))
dest
[[5, 6, 7], None]

In the above sample, the src list is not copied to dest (see None) when I tried to copy and append 4 to it.

dest.append(src[:])
dest
[[5, 6, 7], None, [1, 2, 3]]

As seen in the above snippet, if I add simply add the list (to dest) without any append attempt, it gets inserted.
Any idea?

Answer 1

What append tries to do is to append data on the given list and returns None.

That’s why you will see None at the end of dest.

The following code should do what you want:

dest.append(src + [4])