Extended slice that goes to beginning of sequence with negative stride

Question

Bear with me while I explain my question. Skip down to the bold heading if you already understand extended slice list indexing.

In python, you can index lists using slice notation. Here's an example:

>>> A = list(range(10))
>>> A[0:5]
[0, 1, 2, 3, 4]

You can also include a stride, which acts like a "step":

>>> A[0:5:2]
[0, 2, 4]

The stride is also allowed to be negative, meaning the elements are retrieved in reverse order:

>>> A[5:0:-1]
[5, 4, 3, 2, 1]

But wait! I wanted to see [4, 3, 2, 1, 0]. Oh, I see, I need to decrement the start and end indices:

>>> A[4:-1:-1]
[]

What happened? It's interpreting -1 as being at the end of the array, not the beginning. I know you can achieve this as follows:

>>> A[4::-1]
[4, 3, 2, 1, 0]

But you can't use this in all cases. For example, in a method that's been passed indices.

My question is:

Is there any good pythonic way of using extended slices with negative strides and explicit start and end indices that include the first element of a sequence?

This is what I've come up with so far, but it seems unsatisfying.

>>> A[0:5][::-1]
[4, 3, 2, 1, 0]

Answer 1

It is error-prone to change the semantics of start and stop. Use None or -(len(a) + 1) instead of 0 or -1. The semantics is not arbitrary. See Edsger W. Dijkstra's article "Why numbering should start at zero".

>>> a = range(10)
>>> start, stop, step = 4, None, -1

Or

>>> start, stop, step = 4, -(len(a) + 1), -1
>>> a[start:stop:step]
[4, 3, 2, 1, 0]

Or

>>> s = slice(start, stop, step)
>>> a[s]
[4, 3, 2, 1, 0]

When s is a sequence the negative indexes in s[i:j:k] are treated specially:

If i or j is negative, the index is relative to the end of the string: len(s) + i or len(s) + j is substituted. But note that -0 is still 0.

that is why len(range(10)[4:-1:-1]) == 0 because it is equivalent to range(10)[4:9:-1].

Answer 2

Ok, I think this is probably as good as I will get it. Thanks to Abgan for sparking the idea. This relies on the fact that None in a slice is treated as if it were a missing parameter. Anyone got anything better?

def getReversedList(aList, end, start, step):
    return aList[end:start if start!=-1 else None:step]

edit: check for start==-1, not 0

This is still not ideal, because you're clobbering the usual behavior of -1. It seems the problem here is two overlapping definitions of what's supposed to happen. Whoever wins takes away otherwise valid invocations looking for the other intention.

Answer 3

[ A[b] for b in range(end,start,stride) ]

Slower, however you can use negative indices, so this should work:

[ A[b] for b in range(9, -1, -1) ]

I realize this isn't using slices, but thought I'd offer the solution anyway if using slices specifically for getting the result isn't a priority.

Answer 4

I believe that the following doesn't satisfy you:

def getReversedList(aList, end, start, step):
    if step < 0 and start == 0:
         return aList[end::step]
    return aList[end:start:step]

or does it? :-)

Answer 5

But you can't use that if you are storing your indices in variables for example.

Is this satisfactory?

>>> a = range(10)
>>> start = 0
>>> end = 4
>>> a[4:start-1 if start > 0 else None:-1]
[4, 3, 2, 1, 0]

Answer 6

As you say very few people fully understand everything that you can do with extended slicing, so unless you really need the extra performance I'd do it the "obvious" way:

rev_subset = reversed(data[start:stop])

Answer 7

a[4::-1]

Example:

Python 2.6 (r26:66714, Dec  4 2008, 11:34:15) 
[GCC 4.0.1 (Apple Inc. build 5488)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> a = list(range(10))
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[4:0:-1]
[4, 3, 2, 1]
>>> a[4::-1]
[4, 3, 2, 1, 0]
>>> 

The reason is that the second term is interpreted as "while not index ==". Leaving it out is "while index in range".

Answer 8

I know this is an old question, but in case someone like me is looking for answers:

>>> A[5-1::-1]
[4, 3, 2, 1, 0]

>>> A[4:1:-1]
[4, 3, 2]

Answer 9

You can use a slice(start, stop, step) object, which is such that

s=slice(start, stop, step)
print a[s]

is the same as

print a[start : stop : step]

and, moreover, you can set any of the arguments to None to indicate nothing in between the colons. So in the case you give, you can use slice(4, None, -1).