diff -r deb3e5857d8c Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Thu Oct 27 19:30:10 2016 +0900
+++ b/Objects/unicodeobject.c	Fri Oct 28 01:15:11 2016 +0800
@@ -4703,16 +4703,16 @@
     int kind;
     void *data;
     Py_ssize_t len;
-    PyObject *v;
     int inShift = 0;
-    Py_ssize_t i;
+    Py_ssize_t i, max_char_size;
     unsigned int base64bits = 0;
     unsigned long base64buffer = 0;
-    char * out;
-    char * start;
+    char *out;
+    _PyBytesWriter writer;
 
     if (PyUnicode_READY(str) == -1)
         return NULL;
+
     kind = PyUnicode_KIND(str);
     data = PyUnicode_DATA(str);
     len = PyUnicode_GET_LENGTH(str);
@@ -4720,14 +4720,21 @@
     if (len == 0)
         return PyBytes_FromStringAndSize(NULL, 0);
 
-    /* It might be possible to tighten this worst case */
-    if (len > PY_SSIZE_T_MAX / 8)
+    if (kind == PyUnicode_1BYTE_KIND || kind == PyUnicode_2BYTE_KIND) {
+        max_char_size = 3;
+    } else {
+        assert(kind == PyUnicode_4BYTE_KIND);
+        max_char_size = 6;
+    }
+    if (len > (PY_SSIZE_T_MAX - 2)/ max_char_size) {
         return PyErr_NoMemory();
-    v = PyBytes_FromStringAndSize(NULL, len * 8);
-    if (v == NULL)
-        return NULL;
-
-    start = out = PyBytes_AS_STRING(v);
+    }
+
+    _PyBytesWriter_Init(&writer);
+    out = _PyBytesWriter_Alloc(&writer, len * max_char_size + 2);
+    if (out == NULL)
+        return NULL;
+
     for (i = 0; i < len; ++i) {
         Py_UCS4 ch = PyUnicode_READ(kind, data, i);
 
@@ -4754,7 +4761,7 @@
         else { /* not in a shift sequence */
             if (ch == '+') {
                 *out++ = '+';
-                        *out++ = '-';
+                *out++ = '-';
             }
             else if (ENCODE_DIRECT(ch, !base64SetO, !base64WhiteSpace)) {
                 *out++ = (char) ch;
@@ -4787,13 +4794,13 @@
             base64bits -= 6;
         }
     }
-    if (base64bits)
-        *out++= TO_BASE64(base64buffer << (6-base64bits) );
-    if (inShift)
+    if (inShift) {
+        if (base64bits) {
+            *out++ = TO_BASE64(base64buffer << (6-base64bits));
+        }
         *out++ = '-';
-    if (_PyBytes_Resize(&v, out - start) < 0)
-        return NULL;
-    return v;
+    }
+    return _PyBytesWriter_Finish(&writer, out);
 }
 PyObject *
 PyUnicode_EncodeUTF7(const Py_UNICODE *s,