apacheGH-38768: [Python] Empty slicing an array backwards beyond the …

…start is now empty (apache#40682)

### What changes are included in this PR?

`_normalize_slice` now relies on `slice.indices` (https://docs.python.org/3/reference/datamodel.html#slice.indices).

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Fixing wrong data returned in an edge case.
* GitHub Issue: apache#40642
* GitHub Issue: apache#38768

Lead-authored-by: LucasG0 <guillermou.lucas@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

python/pyarrow/array.pxi

@@ -561,34 +561,7 @@ def _normalize_slice(object arrow_obj, slice key):
  Py_ssize_t start, stop, step
  Py_ssize_t n = len(arrow_obj)
 
- step = key.step or 1
-
- if key.start is None:
- if step < 0:
- start = n - 1
- else:
- start = 0
- elif key.start < 0:
- start = key.start + n
- if start < 0:
- start = 0
- elif key.start >= n:
- start = n
- else:
- start = key.start
-
- if step < 0 and (key.stop is None or key.stop < -n):
- stop = -1
- elif key.stop is None:
- stop = n
- elif key.stop < 0:
- stop = key.stop + n
- if stop < 0: # step > 0 in this case.
- stop = 0
- elif key.stop >= n:
- stop = n
- else:
- stop = key.stop
+ start, stop, step = key.indices(n)
 
  if step != 1:
  indices = np.arange(start, stop, step)

python/pyarrow/tests/test_array.py

@@ -486,6 +486,7 @@ def test_array_slice_negative_step():
  slice(None, None, 2),
  slice(0, 10, 2),
  slice(15, -25, -1), # GH-38768
+ slice(-22, -22, -1), # GH-40642
  ]
 
  for case in cases: