Python: change the first element to the list itself

Question

I have a list:

mylist = [1,2,3]

And I want to change the first element to the list, so a list inside a list. My first try:

mylist + mylist[1:]

gives me

[1, 2, 3, 2, 3] # not what I want

my second try

mylist[0]=mylist

gives me

[[...], 2, 3] # is this an infinite list?

Although I want

[[1,2,3], 2, 3]

Answer 1

mylist[0] = mylist is pointing to reference of the mylist. You can visualise it something like this

Every 1st element is having a list which is again having another list. And this continues.

So the solution to your problem is something like this

>>> mylist = [1,2,3]
>>> mylist[0] = mylist[:]
>>> mylist
[[1, 2, 3], 2, 3]

Answer 2

mylist[0] = mylist gives you a list that contains itself - meaning mylist == mylist[0] == mylist[0][0] == mylist[0][0][0] == .... It sounds like that's not what you want.

From the output you give, you problem is better stated as either:

• Making a new list that contains the old one - mylist = [mylist] + mylist[1:]
• Putting a copy of the list into itself - mylist[0] = mylist[:] mylist[0] = mylist.copy()