Reversing an arbitrary slice in Python

Question

I'm looking for a general method on how to reverse a slice in Python. I read this comprehensive post, which has several nice explanations about how slicing works: Understanding Python's slice notation

Yet I cannot figure out a generalized rule on how to calculate a reversed slice which addresses exactly the same elements in reverse order. I was actually surprised not to find a builtin method doing this.

What I'm looking for is a method reversed_slice that works like this with arbitrary start, stop and step values including negative values:

>>> import numpy as np
>>> a = np.arange(30)
>>> s = np.s_[10:20:2]
>>> a[s]
array([10, 12, 14, 16, 18])
>>> a[reversed_slice(s,len(a))]
array([18, 16, 14, 12, 10])

What I've tried but doesn't work is this:

def reversed_slice(slice_, len_):
    """
    Reverses a slice (selection in array of length len_), 
    addressing the same elements in reverse order.
    """
    assert isinstance(slice_, slice)
    instart, instop, instep = slice_.indices(len_)
    if instep > 0:
        start, stop, step = instop - 1, instart - 1, -instep
    else:
        start, stop, step = instop + 1, instart + 1, -instep
    return slice(start, stop, step)

This works fine for step of 1 and when the last addressed element coincides with stop-1. For other cases it does not:

>>> import numpy as np
>>> a = np.arange(30)
>>> s = np.s_[10:20:2]
>>> a[s]
array([10, 12, 14, 16, 18])
>>> a[reversed_slice(s,len(a))]
array([19, 17, 15, 13, 11])

So it seems like I'm missing some relation like (stop - start) % step. Any help on how to write a general method is greatly appreciated.

Notes:

• I do know that there a other possibilities to get a sequence with the same elements reversed, like calling reversed(a[s]). This is not an option here, as I need to reverse the slice itself. The reason is that I work on h5py datasets which do not allow negative step values in slices.

• An easy but not very elegant way would be the use of coordinate lists, i.e. a[list(reversed(range(*s.indices(len(a)))))]. This is also not an option due to the h5py requirement that indices in the list must be given in increasing order.

Answer 1

You can specify negative values for step.

>>> s = np.s_[20-2:10-2:-2]
>>> a[s]
array([18, 16, 14, 12, 10])

So you can build the reversed_slice function as follows

>>> def reversed_slice(s):
...     """
...     Reverses a slice 
...     """
...     m = (s.stop-s.start) % s.step or s.step
...     return slice(s.stop-m, s.start-m, -s.step)
... 
>>> a = np.arange(30)
>>> s = np.s_[10:20:2]
>>> a[reversed_slice(s)]
array([18, 16, 14, 12, 10])
>>> 
>>> a[reversed_slice(reversed_slice(s))]
array([10, 12, 14, 16, 18])
>>>