diff -r d1c72f5c15bd Doc/c-api/unicode.rst
--- a/Doc/c-api/unicode.rst	Wed Oct 05 22:54:27 2016 -0700
+++ b/Doc/c-api/unicode.rst	Fri Oct 07 02:13:25 2016 +0800
@@ -578,7 +578,7 @@
    Copy characters from one Unicode object into another.  This function performs
    character conversion when necessary and falls back to :c:func:`memcpy` if
    possible.  Returns ``-1`` and sets an exception on error, otherwise returns
-   ``0``.
+   the number of copied characters.
 
    .. versionadded:: 3.3
 
diff -r d1c72f5c15bd Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Wed Oct 05 22:54:27 2016 -0700
+++ b/Lib/test/test_unicode.py	Fri Oct 07 02:13:25 2016 +0800
@@ -4,7 +4,7 @@
 
 (c) Copyright CNRI, All Rights Reserved. NO WARRANTY.
 
-"""#"
+"""
 import _string
 import codecs
 import itertools
@@ -2735,6 +2735,25 @@
             self.assertEqual(unicode_asucs4(s, len(s), 1), s+'\0')
             self.assertEqual(unicode_asucs4(s, len(s), 0), s+'\uffff')
 
+    # Test PyUnicode_CopyCharacters()
+    @support.cpython_only
+    def test_copycharacters(self):
+        from _testcapi import unicode_copycharacters
+        s = 'abcdefghijklmnopqrstuvwxyz'
+        for start in range(len(s)):
+            self.assertEqual(
+                unicode_copycharacters(s, start, 10),
+                (s[start:start+10].ljust(10, '\0'), min(len(s)-start, 10))
+            )
+        self.assertRaises(IndexError, unicode_copycharacters, s, -1, 10)
+        self.assertRaises(IndexError, unicode_copycharacters, s, 30, 10)
+        self.assertRaises(SystemError, unicode_copycharacters, b'', 0, 10)
+        self.assertRaises(SystemError, unicode_copycharacters, s, 0, 11)
+        self.assertRaises(SystemError, unicode_copycharacters, s, 0, -1)
+        for s in ['\xa1\xa2', '\u4f60\u597d', 'a\U0001f600',
+                       'a\ud800b\udfffc', '\ud834\udd1e']:
+            self.assertRaises(SystemError, unicode_copycharacters, s, 0, 10)
+
     @support.cpython_only
     def test_encode_decimal(self):
         from _testcapi import unicode_encodedecimal
diff -r d1c72f5c15bd Modules/_testcapimodule.c
--- a/Modules/_testcapimodule.c	Wed Oct 05 22:54:27 2016 -0700
+++ b/Modules/_testcapimodule.c	Fri Oct 07 02:13:25 2016 +0800
@@ -1859,6 +1859,36 @@
 }
 
 static PyObject *
+unicode_copycharacters(PyObject *self, PyObject *args)
+{
+    PyObject *from, *to;
+    Py_ssize_t start, how_many, copied;
+
+    if (!PyArg_ParseTuple(args, "Onn:unicode_copycharacters", &from,
+                          &start, &how_many)) {
+        return NULL;
+    }
+
+    to = PyUnicode_New(10, 127);
+    if (!to) {
+        return NULL;
+    }
+    assert(PyUnicode_IS_COMPACT_ASCII(to));
+    if (PyUnicode_Fill(to, 0, 127, 0U) < 0) {
+        Py_DECREF(to);
+        return NULL;
+    }
+
+    copied = PyUnicode_CopyCharacters(to, 0, from, start, how_many);
+    if (copied < 0) {
+        Py_DECREF(to);
+        return NULL;
+    }
+
+    return Py_BuildValue("(Nn)", to, copied);
+}
+
+static PyObject *
 unicode_encodedecimal(PyObject *self, PyObject *args)
 {
     Py_UNICODE *unicode;
@@ -4061,6 +4091,7 @@
     {"unicode_aswidechar",      unicode_aswidechar,              METH_VARARGS},
     {"unicode_aswidecharstring",unicode_aswidecharstring,        METH_VARARGS},
     {"unicode_asucs4",          unicode_asucs4,                  METH_VARARGS},
+    {"unicode_copycharacters",  unicode_copycharacters,          METH_VARARGS},
     {"unicode_encodedecimal",   unicode_encodedecimal,           METH_VARARGS},
     {"unicode_transformdecimaltoascii", unicode_transformdecimaltoascii, METH_VARARGS},
     {"unicode_legacy_string",   unicode_legacy_string,           METH_VARARGS},
diff -r d1c72f5c15bd Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Wed Oct 05 22:54:27 2016 -0700
+++ b/Objects/unicodeobject.c	Fri Oct 07 02:13:25 2016 +0800
@@ -1549,15 +1549,19 @@
     if (PyUnicode_READY(to) == -1)
         return -1;
 
-    if (from_start < 0) {
+    if (from_start < 0 || from_start > PyUnicode_GET_LENGTH(from)) {
         PyErr_SetString(PyExc_IndexError, "string index out of range");
         return -1;
     }
-    if (to_start < 0) {
+    if (to_start < 0 || to_start > PyUnicode_GET_LENGTH(to)) {
         PyErr_SetString(PyExc_IndexError, "string index out of range");
         return -1;
     }
-    how_many = Py_MIN(PyUnicode_GET_LENGTH(from), how_many);
+    if (how_many < 0) {
+        PyErr_SetString(PyExc_SystemError, "how_many cannot be negative");
+        return -1;
+    }
+    how_many = Py_MIN(PyUnicode_GET_LENGTH(from)-from_start, how_many);
     if (to_start + how_many > PyUnicode_GET_LENGTH(to)) {
         PyErr_Format(PyExc_SystemError,
                      "Cannot write %zi characters at %zi "