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

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.