bpo-37088: Add the sys.addpendingcall() function.

Doc/library/sys.rst

@@ -43,6 +43,20 @@ always available.
       functions will not see the hook.
 
 
+.. function:: addpendingcall(callback)
+
+   Schedule a *callback* to eventually execute in the main thread.
+
+   A :exc:`RuntimeError` is raised if the *callback* could not be
+   scheduled.
+
+   If *callback* raises an exception it is converted into an
+   *unraisable exception*, see :func:`sys.unraisablehook` for more
+   details.
+
+   .. versionadded:: 3.8
+
+
 .. data:: argv
 
    The list of command line arguments passed to a Python script. ``argv[0]`` is the

Misc/NEWS.d/next/Core and Builtins/2019-05-29-18-47-09.bpo-37088.ZsTtci.rst

@@ -0,0 +1 @@
+Add the sys.addpendingcall() function.

Python/sysmodule.c

@@ -1895,13 +1895,52 @@ sys_getandroidapilevel_impl(PyObject *module)
 #endif   /* ANDROID_API_LEVEL */
 
 
+static int
+_call_pending_pycallback(void *arg)
+{
+    PyObject *callback = (PyObject *)arg;
+    PyObject *res = PyObject_CallFunctionObjArgs(callback, NULL);
+
+    if (res == NULL) {
+        PyErr_WriteUnraisable(callback);
+    }
+    else {
+        Py_DECREF(res);
+    }
+
+    return 0;
+}
+
+
+/*[clinic input]
+sys.addpendingcall
+
+    callback: object
+
+Schedule a *callback* to eventually execute in the main thread.
+[clinic start generated code]*/
+
+static PyObject *
+sys_addpendingcall_impl(PyObject *module, PyObject *callback)
+/*[clinic end generated code: output=8af6dede85199179 input=edaaec4bd3f2c753]*/
+{
+    if (Py_AddPendingCall(&_call_pending_pycallback, (void *)callback)) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "failed to add a pending callback");
+        return NULL;
+    }
+
+    Py_RETURN_NONE;
+}
+
 
 static PyMethodDef sys_methods[] = {
     /* Might as well keep this in alphabetic order */
     SYS_ADDAUDITHOOK_METHODDEF
     {"audit",           (PyCFunction)(void(*)(void))sys_audit, METH_FASTCALL, audit_doc },
     {"breakpointhook",  (PyCFunction)(void(*)(void))sys_breakpointhook,
      METH_FASTCALL | METH_KEYWORDS, breakpointhook_doc},
+    SYS_ADDPENDINGCALL_METHODDEF
     SYS_CALLSTATS_METHODDEF
     SYS__CLEAR_TYPE_CACHE_METHODDEF
     SYS__CURRENT_FRAMES_METHODDEF