Issue #28295: Fixed the documentation and added tests for PyUnicode_AsUCS4().

Original patch by Xiang Zhang.

Modules/_testcapimodule.c

@@ -1829,6 +1829,36 @@ unicode_aswidecharstring(PyObject *self, PyObject *args)
     return Py_BuildValue("(Nn)", result, size);
 }
 
+static PyObject *
+unicode_asucs4(PyObject *self, PyObject *args)
+{
+    PyObject *unicode, *result;
+    Py_UCS4 *buffer;
+    int copy_null;
+    Py_ssize_t str_len, buf_len;
+
+    if (!PyArg_ParseTuple(args, "Unp:unicode_asucs4", &unicode, &str_len, &copy_null)) {
+        return NULL;
+    }
+
+    buf_len = str_len + 1;
+    buffer = PyMem_NEW(Py_UCS4, buf_len);
+    if (buffer == NULL) {
+        return PyErr_NoMemory();
+    }
+    memset(buffer, 0, sizeof(Py_UCS4)*buf_len);
+    buffer[str_len] = 0xffffU;
+
+    if (!PyUnicode_AsUCS4(unicode, buffer, buf_len, copy_null)) {
+        PyMem_FREE(buffer);
+        return NULL;
+    }
+
+    result = PyUnicode_FromKindAndData(PyUnicode_4BYTE_KIND, buffer, buf_len);
+    PyMem_FREE(buffer);
+    return result;
+}
+
 static PyObject *
 unicode_encodedecimal(PyObject *self, PyObject *args)
 {
@@ -3884,6 +3914,7 @@ static PyMethodDef TestMethods[] = {
     {"test_widechar",           (PyCFunction)test_widechar,      METH_NOARGS},
     {"unicode_aswidechar",      unicode_aswidechar,              METH_VARARGS},
     {"unicode_aswidecharstring",unicode_aswidecharstring,        METH_VARARGS},
+    {"unicode_asucs4",          unicode_asucs4,                  METH_VARARGS},
     {"unicode_encodedecimal",   unicode_encodedecimal,           METH_VARARGS},
     {"unicode_transformdecimaltoascii", unicode_transformdecimaltoascii, METH_VARARGS},
     {"unicode_legacy_string",   unicode_legacy_string,           METH_VARARGS},