Address review feedback: trim NEWS wording, generalize the single-call test

Per skirpichev's review on GH-154567:
- Trim the NEWS entry to his suggested wording.
- Move test_byteswap_single_call_result from ComplexDoubleTest into the
  shared CFPTest base class, using self.example and computing
  itemsize/half dynamically instead of hardcoded values, so it now
  covers both ComplexFloatTest ('Zf') and ComplexDoubleTest ('Zd')
  rather than just one.

Author: PhysicistJohn

Lib/test/test_array.py

@@ -1569,6 +1569,22 @@ def test_byteswap(self):
             b.byteswap()
             self.assertEqual(a, b)
 
+    def test_byteswap_single_call_result(self):
+        # A single byteswap() must swap each item's two halves (real,
+        # imag) independently. test_byteswap above only checks that
+        # byteswap() twice round-trips to the original, which passes
+        # even if a single call scrambles multi-item arrays.
+        a = array.array(self.typecode, self.example)
+        original = a.tobytes()
+        a.byteswap()
+        itemsize = a.itemsize
+        half = itemsize // 2
+        expected = bytearray()
+        for i in range(0, len(original), itemsize):
+            item = original[i:i + itemsize]
+            expected += item[half - 1::-1] + item[itemsize - 1:half - 1:-1]
+        self.assertEqual(a.tobytes(), bytes(expected))
+
 
 class HalfFloatTest(FPTest, unittest.TestCase):
     example = [-42.0, 0, 42, 1e2, -1e4]
@@ -1611,20 +1627,6 @@ class ComplexDoubleTest(CFPTest, unittest.TestCase):
     typecode = 'Zd'
     minitemsize = 16
 
-    def test_byteswap_single_call_result(self):
-        # A single byteswap() must swap each 16-byte item's two 8-byte
-        # halves (real, imag) independently. The pre-existing test only
-        # checked that byteswap() twice round-trips to the original,
-        # which passes even if a single call scrambles multi-item arrays.
-        a = array.array(self.typecode, [1+2j, 3+4j, 5+6j])
-        original = a.tobytes()
-        a.byteswap()
-        expected = bytearray()
-        for i in range(0, len(original), 16):
-            item = original[i:i + 16]
-            expected += item[7::-1] + item[15:7:-1]
-        self.assertEqual(a.tobytes(), bytes(expected))
-
 
 class LargeArrayTest(unittest.TestCase):
     typecode = 'b'

Misc/NEWS.d/next/Library/2026-07-23-20-00-00.gh-issue-154566.byteswap-zd.rst

@@ -1,6 +1,4 @@
 Fix :meth:`array.array.byteswap` corrupting data for ``'Zd'`` (complex
 double) arrays with more than one element: the 16-byte item loop advanced
 the buffer pointer by only 8 bytes per iteration, causing items after the
-first to be scrambled. A single :meth:`!byteswap` call produced incorrect
-results while a double call happened to round-trip back to the original
-value, which is why the existing test suite did not catch it.
+first to be scrambled.