apacheGH-38768: [Python] Slicing an array backwards beyond the start …

…now includes first item. (apache#39240)

### What changes are included in this PR?

Minor changes in `_normalize_slice` so `start` and `stop` are both computed in a single if/else block instead of having them modified later in case of a negative `step`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Fixing wrong data returned in an edge case.
* Closes: apache#38768

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

python/pyarrow/array.pxi

@@ -549,32 +549,36 @@ def _normalize_slice(object arrow_obj, slice key):
         Py_ssize_t start, stop, step
         Py_ssize_t n = len(arrow_obj)
 
-    start = key.start or 0
-    if start < 0:
-        start += n
+    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 start >= n:
+    elif key.start >= n:
         start = n
+    else:
+        start = key.start
 
-    stop = key.stop if key.stop is not None else n
-    if stop < 0:
-        stop += n
-        if stop < 0:
+    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 stop >= n:
+    elif key.stop >= n:
         stop = n
+    else:
+        stop = key.stop
 
-    step = key.step or 1
     if step != 1:
-        if step < 0:
-            # Negative steps require some special handling
-            if key.start is None:
-                start = n - 1
-
-            if key.stop is None:
-                stop = -1
-
         indices = np.arange(start, stop, step)
         return arrow_obj.take(indices)
     else:

python/pyarrow/tests/test_array.py

@@ -485,6 +485,7 @@ def test_array_slice_negative_step():
         slice(10, 2, -2),
         slice(None, None, 2),
         slice(0, 10, 2),
+        slice(15, -25, -1),  # GH-38768
     ]
 
     for case in cases: