List slicing with dynamic index on [:index]

Question

I need to slice a list using negative dynamic indexes ([:-index]). This was easy until I realized that if the value of my dynamic index was 0, no items were returned, instea

Answer 1

Another potential solution for fun.

>>> arr = [1, 2, 3]
>>> index = 0
>>> arr[:-index or None]
[1, 2, 3]
>>> index = 1
>>> arr[:-index or None]
[1, 2]

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For higher performance on immutable sequence types like strings, you can avoid slicing the sequence entirely in the case that index is 0 by checking the value of index before the slice operation.

Here's three functions to test in terms of performance:

def shashank1(seq, index):
    return seq[:-index or None]

def shashank2(seq, index):
    return index and seq[:-index] or seq

def shashank3(seq, index):
    return seq[:-index] if index else seq

The latter two should be much faster in the case where index is 0, but may be slower (or faster) in other cases.

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Updated benchmark code: http://repl.it/oA5

Note: The results depend quite a bit on the Python implementation.