python · PhysicistJohn · Jul 24, 2026 · skirpichev · Jul 24, 2026 · skirpichev
diff --git a/Lib/test/test_array.py b/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.
-        # 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.
+        # halves (real, imag) independently.
-        # 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.
+        # halves (real, imag) independently.
+        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'

diff --git a/Misc/NEWS.d/next/Library/2026-07-23-20-00-00.gh-issue-154566.byteswap-zd.rst 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.
-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.
+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.
-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.
+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.
diff --git a/Modules/arraymodule.c b/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];