Why is the list value not reflected when I change the value by slicing the 2D list?

I created 2D list and changed the value of the list by slicing the list.

mylist=[[1,3,5],[3,5,7],[5,7,9]]
mylist[1:2]=[300,400]
mylist

I expected mylist to return [[1,3,5],[300,400],[5,7,9]], but it returned [[1,3,5],300,400,[5,7,9]].

Can anyone explain this?

Thanks.

You needed an extra pair of []

Do this to get what you wanted:

mylist=[[1,3,5],[3,5,7],[5,7,9]]
mylist[1:2]=[[300,400]]
mylist
print(mylist)

Reasons:

To put a single value into a single position in an array, do this:

mylist=[[1,3,5],[3,5,7],[5,7,9]]
mylist[1]=300
mylist
print(mylist)

To put a list into a single position in an array, do this:

mylist=[[1,3,5],[3,5,7],[5,7,9]]
mylist[1]=[300,400]
mylist
print(mylist)

You were telling it to put a list of items into a list of positions

This piece of code highlighted describes a list of positions, albeit containing only one position, namely [1].

mylist=[[1,3,5],[3,5,7],[5,7,9]]

###########
mylist[1:2]=[300,400]
###########

mylist
print(mylist)

Python was doing what it should when you tell it to replace a list of x positions with a list of y values: delete x positions and insert y positions, i.e. change the length of the list if x != y.

What you need to do was to tell it to replace a list of positions with a list of length one (that happened to contain a list of two values).

Thus you needed the right hand side to be a list of length one:

            #############
mylist[1:2]=[ [300,400] ]
            #############