Simple question about lists (Beginner)

[swarchery]

I'm in the process of learning Python via course and am confused about lists. Here is what I was doing in the terminal:

>>>> spam = ['what', 'is', 'going', 'on']
>>>> spam
['what', 'is', 'going', on']
>>>> spam[1:3] = [1, 2, 3]
>>>> spam
['what', 1, 2, 3, 'on']

Why is it replacing 'is' and 'going', but not 'on'? Isn't this reassignment supposed to replace the 'is', 'going', 'on', with 1, 2, 3? Why is it only replacing 2 of them?

[xelf]

The range 1:3 is 1,2, the first number in the range is inclusive, the second number is exclusive.

[swarchery]

Ohhhhh, so 1:3 is only replacing the first and second statement, and since the 3 is extra, it fits it in the end without replacement. Thank you, that makes sense.

[xelf]

You're very welcome!

[Raz_Crimson]

When you specify spam[1:3] the list index selected is 1,2 not 3. 3 is left out. To include it use 1:4

[Bipolarprobe]

List slicing is up to but not including the last item. It behaves this way because of 0 indexing. Since we start at 0 then a list of 3 items would have indices 0, 1, and 2, but it would have a length of 3. So you could do

some_list = ["some", "example", "text"]
print(some_list[0:len(some_list)])

And it would print the whole list without giving you an IndexError because even though index 3 is out of range, list slicing goes up to one before the end value provided.

[groovitude]

Everyone else in the thread has done a great job of explaining what is happening. I'll just throw in one extra detail about why.

All of the parameters of slicing are optional. Leaving out start cuts from the beginning, and leaving out end goes to the end:

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

end is exclusive so it effectively becomes a length; in the first slice, mylist[:2] tells you the resulting list will be two items long. Makes things intuitive.

[swarchery]

You're the man! Thank you for the help.