Slice with negative step inconsistent with python result

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
rust
Domain
data

Research direction

Start with the Si documentation linked in the issue and the slice macro behavior exercised by the Rust example. Compare negative-step defaults against the Python cases, then determine whether the documented semantics or implementation should change; done means the behavior is decided and either corrected or clearly documented with regression coverage.

Written by the indexing model from the issue text.

Description

First of all, I understand that this crate is not meant to reproduce all feature in python, but since in the docs about Si python slice is used as comparison, so I believe this is worth to bring up.

Here is some test case with ndarray:

#[macro_use(s)]
extern crate ndarray;
use ndarray::prelude::*;


fn main() {
    let x: Array1<usize> = Array::zeros(16);

    println!("{}", x.slice(s![..4;1]).len());
    println!("{}", x.slice(s![4..;1]).len());
    println!("{}", x.slice(s![..-4;1]).len());
    println!("{}", x.slice(s![-4..;1]).len());
    println!("{}", x.slice(s![..4;-1]).len());
    println!("{}", x.slice(s![4..;-1]).len());
    println!("{}", x.slice(s![..-4;-1]).len());
    println!("{}", x.slice(s![-4..;-1]).len());
}

which return:

4
12
12
4
4
12
12
4

and in the corresponding python test case (in both python 2.7.6 and python 3.4.3):

def main():
    x = [0] * 16
    print(len(x[:4:1]))
    print(len(x[4::1]))
    print(len(x[:-4:1]))
    print(len(x[-4::1]))
    print(len(x[:4:-1]))
    print(len(x[4::-1]))
    print(len(x[:-4:-1]))
    print(len(x[-4::-1]))


if __name__ == '__main__':
    main()

which return:

4
12
12
4
11
5
3
13

It seems that, with negative step, the default begin is n - 1 and the default end is 0 for python. To be honest, I prefer the ndarray approach to kept the default consistance, but I believe this should either mention in the docs or changed.

Dominant language
Rust
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