I'm trying to get a better grip on Python and I've come across something that feels inconsistent. I have a list called `nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]`. When I use a slice without a step, like `nums[1:-1]`, it works fine and gives me `[1, 2, 3, 4, 5, 6, 7, 8]`. Using a positive step works too, as in `nums[1:-1:2]`, which returns `[0, 2, 4, 6, 8]`. However, when I try to slice with a negative step, such as `nums[1:-1:-2]`, I just get an empty list `[]`. Interestingly, negative stepping still works when I do it in this format: `nums[::-2]`, giving me `[9, 7, 5, 3, 1]`. It seems like the only way to slice with a negative step properly is to do this long-winded workaround: `nums[1:-1][::-2]`, which gives me `[8, 6, 4, 2]`. Can someone help me understand why `nums[1:-1:-2]` returns an empty list?
4 Answers
When slicing with a negative step, you need to remember that the order of start and stop indexes is reversed. For example, with `nums[1:-1:-2]`, you're trying to go from index 1 to index -1, which is not valid for a negative step because 1 is less than -1. Instead, your starting point needs to be higher than your stopping point. If you want to slice in reverse, a proper example would be `nums[3:0:-2]` to get `[3, 1]`.
The issue here is that when you use a negative step, the starting index (which is 1 in your case) has to be higher than the stopping index (which is -1). So you need to start slicing from the end backwards. Try using `nums[-1:1:-2]` instead. This will give you the results you're looking for because you're starting at the highest index and moving backwards.
It's a common misconception! What you're encountering isn't a bug but rather a behavior that comes from how Python interprets slicing. Just keep practicing, and you'll get the hang of it. The important thing to take away is that when slicing backwards, the slicing mechanism expects a certain order, otherwise it just returns an empty list.
Python has its quirks, and this behavior can definitely trip you up if you're not familiar with how negative slicing works. Just think of the slicing rule: with a positive step, start should be less than stop; with a negative step, start must be greater than stop. That's the key to making it work! But you're not alone; many coders stumble on that part.
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