What does [None]* means in python

Question

I was recently studying someone's code and a portion of code given below

class Node:
def __init__(self, height=0, elem=None):
    self.elem = elem
    self.next = [None] * height

What does it mean by [None] * height in the above code

I know what does * operator (as multiplication and unpacking) and None means in python but this is somehow different.

This simply generates a list which all the elements are None and the length of the list is height, when height equals to 0, it returns an empty list. — Jasper Zhou, Nov 09 '19 at 07:47

score 3 · Answer 1 · answered Nov 09 '19 at 07:46

3

It means a list of Nones with a height number of elements. e.g., for height = 3, it is this list:

[None, None, None]

answered Nov 09 '19 at 07:46

mshsayem

score 1 · Accepted Answer · answered Nov 09 '19 at 07:48

1

If you do -

[element] * 3

You get -

[element, element, element]

That's what the code does, [None] * height

That is, if -

height = 4
[None] * height 
# equals [None, None, None, None]

answered Nov 09 '19 at 07:48

Sushant

so you mean that `self.next` is just a list of None? – Shaida Muhammad Nov 09 '19 at 07:50
1

When it is initialised, yes. If height is passed to be more than 0, you will get a list with number of elements, all None equal to the magnitude of height – Sushant Nov 09 '19 at 07:51

score 0 · Answer 3 · answered Nov 09 '19 at 07:45

0

>>> [None]  * 5
[None, None, None, None, None]

Gives you a list of size height in your case

answered Nov 09 '19 at 07:45

bhaskarc

3 Answers3