Question

Normally to reverse a list one should do the following:

>>>>l = [1, 2, 3, 4, 5]
>>>>l[::-1]  
[5, 4, 3, 2, 1]

But isn’t the exact syntax like this:

list[<start>:<end>:<stop>]

and if I don’t give an optional parameter the defaults are as follows(Correct me if I am wrong:

<start> = 0(Beginning of list)
<end>   = 5(Length of list)
<step>  = 1

So if I give the optional parameters, it should in effect produce the same result:

>>>>l[0:5:-1]
>>>>[]

But instead I get an empty list(and I know why this happening), but what are the default values being taken by python in the 1st case?It should take 0 and 5 as default and produce nothing, or is [::-1] different from list[start:end:stop]

Answer 1

l[::-1] is equivalent to l[slice(None,None,-1)], where slice([start], stop[, step]) returns a slice object.
Consider the following:

>>> slice(None,None,-1).indices(5)
(4, -1, -1)
>>> slice(None,None,1).indices(5)
(0, 5, 1)

This is the exact behaviour you see when using the extended indexing syntax to access your list, i.e l[start:stop:step].

>>> l[::-1]
[5, 4, 3, 2, 1]
>>> l[slice(None,None,-1)]
[5, 4, 3, 2, 1]

---

… but what are the default values being taken by python in the 1st case?

The values for start/stop (if not provided) is based on the number if indices (length of list) as well as the value of step. Specifically, see the CPython source for the slice object (Objects/sliceobject.c)

if (r->start == Py_None) {
    *start = *step < 0 ? length-1 : 0;
} else {
    /* .... */
}

if (r->stop == Py_None) {
    *stop = *step < 0 ? -1 : length;
} else {
    /* .... */
}