How to use variables for slicing Python lists?

Question

I have got a harmless list "mylist" of six items. Never empty, never any beastly items.

mylist = ['aaa', 'bbb', 'ccc', 'ddd', 'eee', 'fff']

I found that there are two ways to index a list for slicing purposes. For this question I shall call them upper index (from 0 to 5) and lower index (from -1 to -6). Here is a quick diagram (I had posted a photo with colors, which was replaced by another user):

 <<<<|_________mylist[5:?:-1]_____________|

     |___mylist[0:3:1]______|>>>>>>
 ?
(x)   0      1      2      3      4      5     (6)

     aaa    bbb    ccc    ddd    eee    fff

(-7) -6     -5     -4     -3     -2     -1     (y)
                                                ?
             <<<<<<|___mylist[-1:-5:-1]___|

     |______mylist[-6:?:1]________________|>>>>>

For easily slicing at the start of my list, I can use the upper index like this:

>>> mylist[0:3:1]
['aaa', 'bbb', 'ccc']

For easily slicing near the end, I find the lower index helpful, like:

>>> mylist[-1:-5:-1]
['fff', 'eee', 'ddd', 'ccc']

By "easily" I mean for example the aspect of not concerning my code with the length of the list.

I learnt that Python slicing is "always holding back" (also known as "up to but not including"). I am showing this in my diagram as the "arrow-heads" pointing from the end of a slice to the "next item" ahead ("ahead" in the sense/direction of the slicing).

For my users I need to show the first so many items from my list. (Those are the results from a recursive search and evaluate and sort run.) Because normally this list is rather long, I do print it backwards (in the console), so that when the script ends, the best n results will still be visible on the screen. So my step is -1 in this context.

I would like to use variables, to allow for different needs for different users (and different screen sizes). For debugging and for certain users I want to be able to print all results, i.e. the entire list backwards. I would like something in this syntax:

start = 5     # I only need to know "5" for debugging, normally I can use
              # any small number to show a few selected items of mylist

end = x       # this is my main question, see photo

mylist[start:end:-1]

My question is please, how do I write the first(topmost) slice in the diagram, by using variables and by using the upper list of indices? Or in other words, what is the numerical value of x (and y) in the diagram?

Semi-solutions that I do not want to use:

mylist[5::-1]       # no variable at all

mylist[5:None:-1]   # does not fit in my normal function for printing

I have not managed to use None in any simple operations with my variables, it gives me errors like:

    end = None - 20 
TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'

And I have not managed to convert None to any integer or other number. Is there a way? Would probably be a sacrilege in Pythonland...

If I cannot find the secret number of x, then I might need to work with this:

mylist[-1:-7:-1]  # I want to use the upper index rather

But this approach involves checking the length of mystring.

end = -1 * (len(mystring)+1)  # extra-explicit as an example
returns  -7

mystring[-1:end:-1]

returns  ['fff', 'eee', 'ddd', 'ccc', 'bbb', 'aaa']  # this is the output I want, just not the method

What I have done before asking you:
I have written a testing-script and have tried guessing x. I have searched and read up plenty on slicing and variables here on Stack Overflow (like when use negative number to slice a string in Python, 0 is disabled?) and online (like https://docs.python.org/3/library/functions.html?highlight=slice#slice) and I searched through ebooks like Introducing Python and Learning Python.

A plea:
Please do not tell me that what I want is wrong. I can slice mylist[0:6:1] although there is no item indexed 6 in my example list, same for -7. This is why I have this hunch that there might be numbers for x and for y too, which I could use in my variables.

You are welcome to tell me that it does not exist or cannot be done the way I would prefer. In the latter case, I am also asking you for alternatives, as close to my request as possible.

Background for those who want more:
This question is more about "figuring out slicing" than about making the printout possible. I do have workarounds, but would love to find out more.

Answer 1

The short answer is: No.

---

You have to use None if you don't want to calculate the length of the list (although that's an O(1) operation in Python):

>>> mylist = ['aaa', 'bbb', 'ccc', 'ddd', 'eee', 'fff']
>>> mylist[2:None:-1]  # first 3 items backwards (note off-by-one indexing)
['ccc', 'bbb', 'aaa']
>>> mylist[None:None:-1]  # all items backwards
['fff', 'eee', 'ddd', 'ccc', 'bbb', 'aaa']

---

You could use a placeholder for "the whole thing", e.g. 0:

>>> end = 3
>>> mylist[end - 1 if end else None::-1]
['ccc', 'bbb', 'aaa']
>>> end = 0
>>> mylist[end - 1 if end else None::-1]
['fff', 'eee', 'ddd', 'ccc', 'bbb', 'aaa']

---

You could also consider two steps, i.e. take the slice you want then reverse it:

>>> mylist[:3][::-1]
['ccc', 'bbb', 'aaa']

Answer 2

Just make a default value and if the user changes the default (max) then change your variable.

>>> mylist = ['aaa', 'bbb', 'ccc', 'ddd', 'eee', 'fff']
>>> display_num = len(mylist)
>>> mylist[display_num-1::-1]
['fff', 'eee', 'ddd', 'ccc', 'bbb', 'aaa']

Change the display_num variable to the number of results you want to show.

>>> display_num = 4
>>> mylist[display_num-1::-1]
['ddd', 'ccc', 'bbb', 'aaa']

I think your options are None or the length of the list.

... or just use a try catch

>>> display_num = None
>>> try:
>>>     mylist[display_num-1::-1]
>>> except TypeError:
>>>     mylist[::-1]

Answer 3

If I understood your question correctly, you want y such that

mylist[2:y:-1] == ['ccc', 'bbb', 'aaa']

where y must be an integer.

The y you are looking for is -len(mylist)-1 or a lower integer. If you need a constant, you could use -sys.maxsize (as it is impossible to have lists larger than maxsize).

---

If we look at CPython's source code, we can see that list objects internally use the function PySlice_GetIndicesEx. In order to do what you want, PySlice_GetIndicesEx must set step = -1, start = 2 and slicelength = 3.

start = 2 and step = -1 are already set by your code. Are there values for y that can lead to slicelength = 3? Let's see...

When step is negative, the line that calculates slicelength is this one:

*slicelength = (*stop-*start+1)/(*step)+1;

From here, we can see that in order to have slicelength = (stop - start + 1) / step + 1 = 3, we must have stop = -1. However, the case stop < 0 is treated specially:

if (*stop < 0) *stop += length;
if (*stop < 0) *stop = (*step < 0) ? -1 : 0;

So, if we provide a negative stop, it gets increased by len(mylist). If stop is still negative it is set to -1, which is the value we need.