gh-155621: Fix unrepresentable defaults in socket.sendmsg() signature

Lib/test/test_socket.py

@@ -12,6 +12,7 @@
 import errno
 import fractions
 import gc
+import inspect
 import io
 import itertools
 import math
@@ -945,6 +946,14 @@ def requireSocket(*args):
 
 class GeneralModuleTests(unittest.TestCase):
 
+    @unittest.skipUnless(hasattr(socket.socket, "sendmsg"),
+                         "sendmsg not supported")
+    def test_sendmsg_signature(self):
+        self.assertEqual(
+            str(inspect.signature(socket.socket.sendmsg)),
+            "(self, buffers, ancdata=(), flags=0, address=None, /)",
+        )
+
     @unittest.skipUnless(_socket is not None, 'need _socket module')
     def test_socket_type(self):
         self.assertTrue(gc.is_tracked(_socket.socket))

Misc/NEWS.d/next/Library/2026-08-12-20-51-27.gh-issue-155621.h9V1sW.rst

@@ -0,0 +1,3 @@
+Fix the signature of :meth:`socket.socket.sendmsg` to expose the *ancdata*
+and *address* defaults as ``()`` and ``None`` instead of
+``<unrepresentable>``.

Modules/socketmodule.c

@@ -4929,9 +4929,9 @@ sock_sendmsg_impl(PySocketSockObject *s, void *data)
 _socket.socket.sendmsg
     self as s: self(type="PySocketSockObject *")
     buffers as data_arg: object
-    ancdata as cmsg_arg: object = NULL
+    ancdata as cmsg_arg: object(c_default="NULL") = ()
     flags: int = 0
-    address as addr_arg: object = NULL
+    address as addr_arg: object(c_default="NULL") = None
     /
 
 Send normal and ancillary data to the socket.
@@ -4955,7 +4955,7 @@ static PyObject *
 _socket_socket_sendmsg_impl(PySocketSockObject *s, PyObject *data_arg,
                             PyObject *cmsg_arg, int flags,
                             PyObject *addr_arg)
-/*[clinic end generated code: output=3b4cb1110644ce39 input=8ae408971a3aa329]*/
+/*[clinic end generated code: output=3b4cb1110644ce39 input=5fdf9b49bed3848b]*/
 
 {
     Py_ssize_t i, ndatabufs = 0, ncmsgs, ncmsgbufs = 0;