gh-154566: Fix array.byteswap() corrupting 'Zd' arrays with more than one element

The 16-byte-item loop in array_array_byteswap_impl() advanced the
buffer pointer by only 8 bytes per iteration, even though each
iteration swaps a full 16-byte item (two independent 8-byte halves
for the real/imaginary double components). This caused every item
after the first to overlap the previous iteration's byte window and
come out scrambled.

A single byteswap() call gave wrong bytes for the second item onward.
Calling byteswap() twice happened to round-trip back to the original
value (the corruption is self-canceling under double application),
which is why the existing test suite -- which only checked a
double-call round-trip -- didn't catch it. Added a regression test
that checks the actual byte-level result of a single call.

Author: PhysicistJohn

Lib/test/test_array.py

@@ -1611,6 +1611,20 @@ 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

@@ -0,0 +1,6 @@
+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.

Modules/arraymodule.c

@@ -1617,7 +1617,7 @@ array_array_byteswap_impl(arrayobject *self)
         break;
     case 16:
         assert(strcmp(self->ob_descr->typecode, "Zd") == 0);
-        for (p = self->ob_item, i = Py_SIZE(self); --i >= 0; p += 8) {
+        for (p = self->ob_item, i = Py_SIZE(self); --i >= 0; p += 16) {
             char t0 = p[0];
             char t1 = p[1];
             char t2 = p[2];