gh-154568: Fix array unpickling of little-endian float16 (#154569)

PhysicistJohn · web-flow · commit f3be08aa6f6f · 2026-08-11T15:41:05.000+02:00
Fix array._array_reconstructor() ignoring requested byte order for float16.

The slow-path decoder for IEEE_754_FLOAT16_LE/BE computed the
byte-order flag by comparing mformat_code against IEEE_754_FLOAT_LE
(the 32-bit float constant) instead of IEEE_754_FLOAT16_LE. Since the
float16 mformat codes are never equal to that constant, the comparison
was always false, so the decoder always treated input as big-endian
regardless of what was requested.

Adds a regression test.
diff --git a/Lib/test/test_array.py b/Lib/test/test_array.py
@@ -209,6 +209,10 @@ def test_numbers(self):
              [-1<<63, (1<<63)-1, 0]),
             (['l'], SIGNED_INT64_BE, '>qqq',
              [-1<<63, (1<<63)-1, 0]),
+            (['e'], IEEE_754_FLOAT16_LE, '<eeee',
+             [1.0, float('inf'), float('-inf'), -0.0]),
+            (['e'], IEEE_754_FLOAT16_BE, '>eeee',
+             [1.0, float('inf'), float('-inf'), -0.0]),
             (['f'], IEEE_754_FLOAT_LE, '<ffff',
              [16711938.0, float('inf'), float('-inf'), -0.0]),
             (['f'], IEEE_754_FLOAT_BE, '>ffff',
@@ -239,6 +243,16 @@ def test_numbers(self):
                 self.assertEqual(a, b,
                     msg="{0!r} != {1!r}; testcase={2!r}".format(a, b, testcase))
 
+    def test_float16_endianness(self):
+        # gh-154568: array_reconstructor() slow-path decoder for
+        # IEEE_754_FLOAT16_LE ignored the encoding.
+        le_bytes = struct.pack('<e', 1.5)
+        be_bytes = struct.pack('>e', 1.5)
+        b_le = array_reconstructor(array.array, 'd', IEEE_754_FLOAT16_LE, le_bytes)
+        b_be = array_reconstructor(array.array, 'd', IEEE_754_FLOAT16_BE, be_bytes)
+        self.assertEqual(b_le.tolist(), [1.5])
+        self.assertEqual(b_be.tolist(), [1.5])
+
     def test_unicode(self):
         teststr = "Bonne Journ\xe9e \U0002030a\U00020347"
         testcases = (
diff --git a/Misc/NEWS.d/next/Library/2026-07-23-21-00-00.gh-issue-154568.float16-reconstructor.rst b/Misc/NEWS.d/next/Library/2026-07-23-21-00-00.gh-issue-154568.float16-reconstructor.rst
@@ -0,0 +1 @@
+Fix :mod:`array` unpickling of little-endian float16.
diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
@@ -2281,7 +2281,7 @@ array__array_reconstructor_impl(PyObject *module, PyTypeObject *arraytype,
     case IEEE_754_FLOAT16_LE:
     case IEEE_754_FLOAT16_BE: {
         Py_ssize_t i;
-        int le = (mformat_code == IEEE_754_FLOAT_LE) ? 1 : 0;
+        int le = (mformat_code == IEEE_754_FLOAT16_LE) ? 1 : 0;
         Py_ssize_t itemcount = Py_SIZE(items) / 2;
         const char *memstr = PyBytes_AS_STRING(items);
 

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Fix :mod:`array` unpickling of little-endian float16.