What does extended slice syntax actually do for negative steps?
This question already has an answer here:
[-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'
The Python documentation (here's the technical one; the explanation for range() is a bit easier to understand) is more correct than the simplified "every kth element" explanation. The slicing parameters are aptly named
slice[start:stop:step]
so the slice starts at the location defined by start , stops before the location stop is reached, and moves from one position to the next by step items.
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