What are the default values set to in a slice?

Question

I have had a look at the top answers on why-does-list-1-not-equal-listlenlist-1 and what-are-the-default-slice-indices-in-python-really to learn about what the default values are set to in a slice. Both of the top answers refer to the following documentation:

Given s[i:j:k],

If i or j are omitted or None, they become “end” values (which end depends on the sign of k). Note, k cannot be zero. If k is None, it is treated like 1.

Suppose I have the following code:

s = "hello"
forward_s = s[::1]
print(forward_s) # => "hello"

backward_s = s[::-1]
print(backward_s) # => "olleh"

I know that if the indices are omitted then Python treats that as if the value of None was used in those places. According to the documentation the indices of i and j in [i:j:k] are set to "end" values depending on the sign of k. For a positive k, I'm assuming i is set to 0 and j is set to the length of the string. What are the values set for a negative k though?

For instance, the following also reverses the string:

reversed_str = s[-1:-6:-1]

So maybe the default value for i is set to -1 and j is set to -len(s) - 1 when k = -1?

Answer 1

The default values are None, None, and None.

class Sliceable(object):
    def __getitem__(self, slice):
        print(slice)

Sliceable()[::]
>>> slice(None, None, None)

This is regardless of the fact, that slice() does require a stop argument. The library documentation is a little bit less explicit on this, but the C API makes clear, that all three values maybe empty:

The start, stop, and step parameters are used as the values of the slice object attributes of the same names. Any of the values may be NULL, in which case the None will be used for the corresponding attribute.

It's up to the sliceable object to make sense of the default values. The convention is to use first element, past last element, minimal stepping as implemented by the builtin collections:

l = [1, 2, 3]
l[slice(None, None, None])
>>> [1, 2, 3]
s[None:None:None])
>>> [1, 2, 3]

Negative stepping will cause the default start and end values to be reversed semantically, i.e.:

s = 'abc'
s[slice(None, None, -1)]
>>> 'cba'
s[::-1]
>>> 'cba'

Note that this does not mean a simple value flip, the default value of end is typically "one past the end of the sequence, in whatever direction", since range() is not inclusive for the end value, but the default values for a slice should include the full sequence.

This is documented here:

s[i:j:k]

The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that 0 <= n < (j-i)/k. In other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j). When k is positive, i and j are reduced to len(s) if they are greater. When k is negative, i and j are reduced to len(s) - 1 if they are greater. If i or j are omitted or None, they become “end” values (which end depends on the sign of k). Note, k cannot be zero. If k is None, it is treated like 1.