bpo-43916: PyStdPrinter_Type uses Py_TPFLAGS_DISALLOW_INSTANTIATION (GH-25749)

The PyStdPrinter_Type type now uses the
Py_TPFLAGS_DISALLOW_INSTANTIATION flag to disallow instantiation,
rather than seting a tp_init method which always fail.

Write also unit tests for PyStdPrinter_Type.

Lib/test/test_embed.py

@@ -1473,11 +1473,67 @@ class AuditingTests(EmbeddingTestsMixin, unittest.TestCase):
                                       timeout=support.SHORT_TIMEOUT,
                                       returncode=1)
 
+
 class MiscTests(EmbeddingTestsMixin, unittest.TestCase):
     def test_unicode_id_init(self):
         # bpo-42882: Test that _PyUnicode_FromId() works
         # when Python is initialized multiples times.
         self.run_embedded_interpreter("test_unicode_id_init")
 
+
+class StdPrinterTests(EmbeddingTestsMixin, unittest.TestCase):
+    # Test PyStdPrinter_Type which is used by _PySys_SetPreliminaryStderr():
+    #   "Set up a preliminary stderr printer until we have enough
+    #    infrastructure for the io module in place."
+
+    def get_stdout_fd(self):
+        return sys.__stdout__.fileno()
+
+    def create_printer(self, fd):
+        ctypes = import_helper.import_module('ctypes')
+        PyFile_NewStdPrinter = ctypes.pythonapi.PyFile_NewStdPrinter
+        PyFile_NewStdPrinter.argtypes = (ctypes.c_int,)
+        PyFile_NewStdPrinter.restype = ctypes.py_object
+        return PyFile_NewStdPrinter(fd)
+
+    def test_write(self):
+        message = "unicode:\xe9-\u20ac-\udc80!\n"
+
+        stdout_fd = self.get_stdout_fd()
+        stdout_fd_copy = os.dup(stdout_fd)
+        self.addCleanup(os.close, stdout_fd_copy)
+
+        rfd, wfd = os.pipe()
+        self.addCleanup(os.close, rfd)
+        self.addCleanup(os.close, wfd)
+        try:
+            # PyFile_NewStdPrinter() only accepts fileno(stdout)
+            # or fileno(stderr) file descriptor.
+            os.dup2(wfd, stdout_fd)
+
+            printer = self.create_printer(stdout_fd)
+            printer.write(message)
+        finally:
+            os.dup2(stdout_fd_copy, stdout_fd)
+
+        data = os.read(rfd, 100)
+        self.assertEqual(data, message.encode('utf8', 'backslashreplace'))
+
+    def test_methods(self):
+        fd = self.get_stdout_fd()
+        printer = self.create_printer(fd)
+        self.assertEqual(printer.fileno(), fd)
+        self.assertEqual(printer.isatty(), os.isatty(fd))
+        printer.flush()  # noop
+        printer.close()  # noop
+
+    def test_disallow_instantiation(self):
+        fd = self.get_stdout_fd()
+        printer = self.create_printer(fd)
+        PyStdPrinter_Type = type(printer)
+        with self.assertRaises(TypeError):
+            PyStdPrinter_Type(fd)
+
+
 if __name__ == "__main__":
     unittest.main()

Objects/fileobject.c

@@ -325,29 +325,6 @@ typedef struct {
     int fd;
 } PyStdPrinter_Object;
 
-static PyObject *
-stdprinter_new(PyTypeObject *type, PyObject *args, PyObject *kews)
-{
-    PyStdPrinter_Object *self;
-
-    assert(type != NULL && type->tp_alloc != NULL);
-
-    self = (PyStdPrinter_Object *) type->tp_alloc(type, 0);
-    if (self != NULL) {
-        self->fd = -1;
-    }
-
-    return (PyObject *) self;
-}
-
-static int
-stdprinter_init(PyObject *self, PyObject *args, PyObject *kwds)
-{
-    PyErr_SetString(PyExc_TypeError,
-                    "cannot create 'stderrprinter' instances");
-    return -1;
-}
-
 PyObject *
 PyFile_NewStdPrinter(int fd)
 {
@@ -390,7 +367,7 @@ stdprinter_write(PyStdPrinter_Object *self, PyObject *args)
         return NULL;
     }
 
-    /* Encode Unicode to UTF-8/surrogateescape */
+    /* Encode Unicode to UTF-8/backslashreplace */
     str = PyUnicode_AsUTF8AndSize(unicode, &n);
     if (str == NULL) {
         PyErr_Clear();
@@ -507,7 +484,7 @@ PyTypeObject PyStdPrinter_Type = {
     PyObject_GenericGetAttr,                    /* tp_getattro */
     0,                                          /* tp_setattro */
     0,                                          /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT,                         /* tp_flags */
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION, /* tp_flags */
     0,                                          /* tp_doc */
     0,                                          /* tp_traverse */
     0,                                          /* tp_clear */
@@ -523,9 +500,9 @@ PyTypeObject PyStdPrinter_Type = {
     0,                                          /* tp_descr_get */
     0,                                          /* tp_descr_set */
     0,                                          /* tp_dictoffset */
-    stdprinter_init,                            /* tp_init */
+    0,                                          /* tp_init */
     PyType_GenericAlloc,                        /* tp_alloc */
-    stdprinter_new,                             /* tp_new */
+    0,                                          /* tp_new */
     PyObject_Del,                               /* tp_free */
 };
 

Python/sysmodule.c

@@ -3007,7 +3007,7 @@ err_occurred:
 /* Set up a preliminary stderr printer until we have enough
    infrastructure for the io module in place.
 
-   Use UTF-8/surrogateescape and ignore EAGAIN errors. */
+   Use UTF-8/backslashreplace and ignore EAGAIN errors. */
 static PyStatus
 _PySys_SetPreliminaryStderr(PyObject *sysdict)
 {