About slices in Python

Question

1. Is there any difference between mylist[:] and mylist[::]?
2. What's the rationale for mylist[::0] to raise an error since negative steps are allowed?

Answer 1

1. No. Both result in slice(None, None, None).

2. Positive strides go forwards. Negative strides go backwards. Zero strides go... nowhere? How exactly would that work? An infinite sequence of a single value?

• Kimvais: You could perhaps add an explanation about slice objects. At least see help(slice) :)
• Chris Rudd: actually I would expect to get either a None object or a 0 length array back on a 0 length slice

Answer 2

No difference between mylist[:] and mylist[::]
mylist[::0]

This implies from starting index to last index without any step, don't know in what world it would be possible.

• Open AI - Opting Out: The explicit zero makes you think, "No-one would ever do that", but if the value had been calculated instead it wouldn't be possible to spot, and I can imagine it happening.

Answer 3

Third element is for steps. When you write mylist[:] it will assume step will be 1 which is same case in mylist[::].

If you write mylist[::0] then it will raise error because steps can be +ve or -ve not 0

• Ignacio Vazquez-Abrams: They can be 0, it's just that a 0 stride is meaningless.
• warvariuc: @IgnacioVazquez-Abrams, >>> [1,2,3,4,5][::0] ValueError: slice step cannot be zero
• Ignacio Vazquez-Abrams: That exception comes from list, not slice.