Alex Becker Alex Becker - 11 days ago 4
Python Question

What does extended slice syntax actually do for negative steps?

The extended slice syntax in python has been explained to me as "

a[n:m:k] returns every kth element from n to m
".

This gives me a good idea what to expect when k is positive. But I'm lost on how to interpret
a[n:m:k]
for negative k. I know that
a[::-1]
reverses a, and that
a[::-k]
takes ever kth element of the reversed a.

But how is this a generalization of the definition for k positive? I'd like to know how
a[n:m:k]
is actually defined, so that (for example) I can understand why:

"abcd"[-1:0:-1] = "dcb"


Is
a[n:m:-k]
reversing the sequence a, then taking the elements with original indices starting from n and ending one before m or something? I don't think so, because this pattern doesn't fit other values of n and m I've tried. But I'm at a loss to figure out how this is actually defined, and searching has gotten me nowhere.

Answer

[-1:0:-1] means : start from the index len(string)-1 and move up to 0(not included) and take a step of -1(reverse)

So, the following indexes are fetched:

le -1, le-1-1, le-1-1-1  .... 1    # le is len(string)

example:

In [24]: strs='foobar'

In [25]: le=len(strs)

In [26]: strs[-1:0:-1]      #the first -1 is equivalent to len(strs)-1

Out[26]: 'raboo'

In [27]: strs[le-1:0:-1]   
Out[27]: 'raboo'
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