gh-145633: Fix struct.pack('f') on s390x (#146422)

Use PyFloat_Pack4() to raise OverflowError.
Add more tests on packing/unpacking floats.

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
(cherry picked from commit 8de70b3)

Author: vstinner

Lib/test/test_struct.py

@@ -392,6 +392,21 @@ def test_705836(self):
         big = (1 << 25) - 1
         big = math.ldexp(big, 127 - 24)
         self.assertRaises(OverflowError, struct.pack, ">f", big)
+        self.assertRaises(OverflowError, struct.pack, "<f", big)
+        # same for native format, see gh-145633
+        self.assertRaises(OverflowError, struct.pack, "f", big)
+
+        # And for half-floats
+        big = (1 << 11) - 1
+        big = math.ldexp(big, 15 - 10)
+        packed = struct.pack(">e", big)
+        unpacked = struct.unpack(">e", packed)[0]
+        self.assertEqual(big, unpacked)
+        big = (1 << 12) - 1
+        big = math.ldexp(big, 15 - 11)
+        self.assertRaises(OverflowError, struct.pack, ">e", big)
+        self.assertRaises(OverflowError, struct.pack, "<e", big)
+        self.assertRaises(OverflowError, struct.pack, "e", big)
 
     def test_1530559(self):
         for code, byteorder in iter_integer_formats():
@@ -832,6 +847,27 @@ def test_operations_on_half_initialized_Struct(self):
         self.assertRaises(RuntimeError, repr, S)
         self.assertEqual(S.size, -1)
 
+    def test_float_round_trip(self):
+        INF = float('inf')
+        NAN = float('nan')
+
+        for format in (
+            "f", "<f", ">f",
+            "d", "<d", ">d",
+            "e", "<e", ">e",
+        ):
+            with self.subTest(format=format):
+                f = struct.unpack(format, struct.pack(format, 1.5))[0]
+                self.assertEqual(f, 1.5)
+                f = struct.unpack(format, struct.pack(format, NAN))[0]
+                self.assertTrue(math.isnan(f), f)
+                f = struct.unpack(format, struct.pack(format, INF))[0]
+                self.assertTrue(math.isinf(f), f)
+                self.assertEqual(math.copysign(1.0, f), 1.0)
+                f = struct.unpack(format, struct.pack(format, -INF))[0]
+                self.assertTrue(math.isinf(f), f)
+                self.assertEqual(math.copysign(1.0, f), -1.0)
+
 
 class UnpackIteratorTest(unittest.TestCase):
     """

Misc/NEWS.d/next/Library/2026-03-26-11-04-42.gh-issue-145633.RWjlaX.rst

@@ -0,0 +1,2 @@
+Fix ``struct.pack('f', float)``: use :c:func:`PyFloat_Pack4` to raise
+:exc:`OverflowError`. Patch by Sergey B Kirpichev and Victor Stinner.

Modules/_struct.c

@@ -768,14 +768,13 @@ np_halffloat(_structmodulestate *state, char *p, PyObject *v, const formatdef *f
 static int
 np_float(_structmodulestate *state, char *p, PyObject *v, const formatdef *f)
 {
-    float x = (float)PyFloat_AsDouble(v);
+    double x = PyFloat_AsDouble(v);
     if (x == -1 && PyErr_Occurred()) {
         PyErr_SetString(state->StructError,
                         "required argument is not a float");
         return -1;
     }
-    memcpy(p, (char *)&x, sizeof x);
-    return 0;
+    return PyFloat_Pack4(x, p, PY_LITTLE_ENDIAN);
 }
 
 static int