gh-156867: struct.pack() check for overflows for Zf type

Author: skirpichev

Lib/test/test_struct.py

@@ -424,6 +424,16 @@ def test_705836(self):
         self.assertRaises(OverflowError, struct.pack, "<e", big)
         self.assertRaises(OverflowError, struct.pack, "e", big)
 
+    def test_156867(self):
+        big_real = 1e300
+        self.assertRaises(OverflowError, struct.pack, ">Zf", big_real)
+        self.assertRaises(OverflowError, struct.pack, "<Zf", big_real)
+        self.assertRaises(OverflowError, struct.pack, "Zf", big_real)
+        big_imag = 1e300j
+        self.assertRaises(OverflowError, struct.pack, ">Zf", big_imag)
+        self.assertRaises(OverflowError, struct.pack, "<Zf", big_imag)
+        self.assertRaises(OverflowError, struct.pack, "Zf", big_imag)
+
     def test_1530559(self):
         for code, byteorder in iter_integer_formats():
             format = byteorder + code

Misc/NEWS.d/next/Library/2026-09-03-04-32-06.gh-issue-156867.hq3efC.rst

@@ -0,0 +1,3 @@
+Raise :exc:`OverflowError`'s for native ``'Zf'`` format in :func:`struct.pack`,
+like for ``'f'`` format.  Previously overflows in the :c:expr:`float complex`
+type were silent.  Patch by Sergey B Kirpichev.

Modules/_struct.c

@@ -790,15 +790,19 @@ np_float_complex(_structmodulestate *state, char *p, PyObject *v,
                  const formatdef *f)
 {
     Py_complex c = PyComplex_AsCComplex(v);
-    float x[2] = {(float)c.real, (float)c.imag};
 
     if (c.real == -1 && PyErr_Occurred()) {
         PyErr_SetString(state->StructError,
                         "required argument is not a complex");
         return -1;
     }
-    memcpy(p, &x, sizeof(x));
-    return 0;
+
+    int ret = PyFloat_Pack4(c.real, p, PY_LITTLE_ENDIAN);
+
+    if (ret) {
+        return ret;
+    }
+    return PyFloat_Pack4(c.imag, p + sizeof(float), PY_LITTLE_ENDIAN);
 }
 
 static int