diff -r fc7ceb001eec Objects/dictobject.c
--- a/Objects/dictobject.c	Mon Nov 18 21:11:57 2013 +0100
+++ b/Objects/dictobject.c	Mon Nov 18 22:10:39 2013 +0100
@@ -1397,9 +1397,10 @@ static PyObject *
 dict_repr(PyDictObject *mp)
 {
     Py_ssize_t i;
-    PyObject *s, *temp, *colon = NULL;
-    PyObject *pieces = NULL, *result = NULL;
+    PyObject *colon = NULL, *sep = NULL;
     PyObject *key, *value;
+    _PyUnicodeWriter writer;
+    int first;
 
     i = Py_ReprEnter((PyObject *)mp);
     if (i != 0) {
@@ -1407,74 +1408,88 @@ dict_repr(PyDictObject *mp)
     }
 
     if (mp->ma_used == 0) {
-        result = PyUnicode_FromString("{}");
-        goto Done;
+        Py_ReprLeave((PyObject *)mp);
+        return PyUnicode_FromString("{}");
     }
 
-    pieces = PyList_New(0);
-    if (pieces == NULL)
-        goto Done;
+    _PyUnicodeWriter_Init(&writer);
+    writer.overallocate = 1;
+    /* "{" + "1: 2" + ", 3: 4" * (len - 1) + "}" */
+    writer.min_length = 1 + 1 + 6 * (mp->ma_used - 1) + 1;
 
     colon = PyUnicode_FromString(": ");
     if (colon == NULL)
         goto Done;
 
+    if (_PyUnicodeWriter_WriteChar(&writer, '{') < 0)
+        goto Done;
+
     /* Do repr() on each key+value pair, and insert ": " between them.
        Note that repr may mutate the dict. */
     i = 0;
+    first = 1;
     while (PyDict_Next((PyObject *)mp, &i, &key, &value)) {
-        int status;
+        PyObject *s;
+
+        if (!first) {
+            if (sep == NULL) {
+                sep = PyUnicode_FromString(", ");
+                if (sep == NULL)
+                    goto Done;
+            }
+
+            if (_PyUnicodeWriter_WriteStr(&writer, sep) < 0)
+                goto Done;
+        }
+        first = 0;
+
         /* Prevent repr from deleting key or value during key format. */
         Py_INCREF(key);
         Py_INCREF(value);
+
         s = PyObject_Repr(key);
-        PyUnicode_Append(&s, colon);
-        if (s == NULL)
+        if (s == NULL || _PyUnicodeWriter_WriteStr(&writer, s) < 0) {
+            Py_DECREF(key);
+            Py_DECREF(value);
+            Py_XDECREF(s);
             goto Done;
-
-        PyUnicode_AppendAndDel(&s, PyObject_Repr(value));
+        }
+        Py_DECREF(s);
+
+        if (_PyUnicodeWriter_WriteStr(&writer, colon) < 0) {
+            Py_DECREF(key);
+            Py_DECREF(value);
+            goto Done;
+        }
+
+        s = PyObject_Repr(value);
+        if (s == NULL || _PyUnicodeWriter_WriteStr(&writer, s) < 0) {
+            Py_DECREF(key);
+            Py_DECREF(value);
+            Py_XDECREF(s);
+            goto Done;
+        }
+        Py_DECREF(s);
+
         Py_DECREF(key);
         Py_DECREF(value);
-        if (s == NULL)
-            goto Done;
-        status = PyList_Append(pieces, s);
-        Py_DECREF(s);  /* append created a new ref */
-        if (status < 0)
-            goto Done;
     }
-
-    /* Add "{}" decorations to the first and last items. */
-    assert(PyList_GET_SIZE(pieces) > 0);
-    s = PyUnicode_FromString("{");
-    if (s == NULL)
+    Py_CLEAR(colon);
+    Py_CLEAR(sep);
+
+    if (_PyUnicodeWriter_WriteChar(&writer, '}') < 0)
         goto Done;
-    temp = PyList_GET_ITEM(pieces, 0);
-    PyUnicode_AppendAndDel(&s, temp);
-    PyList_SET_ITEM(pieces, 0, s);
-    if (s == NULL)
-        goto Done;
-
-    s = PyUnicode_FromString("}");
-    if (s == NULL)
-        goto Done;
-    temp = PyList_GET_ITEM(pieces, PyList_GET_SIZE(pieces) - 1);
-    PyUnicode_AppendAndDel(&temp, s);
-    PyList_SET_ITEM(pieces, PyList_GET_SIZE(pieces) - 1, temp);
-    if (temp == NULL)
-        goto Done;
-
-    /* Paste them all together with ", " between. */
-    s = PyUnicode_FromString(", ");
-    if (s == NULL)
-        goto Done;
-    result = PyUnicode_Join(s, pieces);
-    Py_DECREF(s);
+
+    Py_ReprLeave((PyObject *)mp);
+
+    return _PyUnicodeWriter_Finish(&writer);
 
 Done:
-    Py_XDECREF(pieces);
+    _PyUnicodeWriter_Dealloc(&writer);
     Py_XDECREF(colon);
+    Py_XDECREF(sep);
     Py_ReprLeave((PyObject *)mp);
-    return result;
+    return NULL;
 }
 
 static Py_ssize_t
diff -r fc7ceb001eec Objects/listobject.c
--- a/Objects/listobject.c	Mon Nov 18 21:11:57 2013 +0100
+++ b/Objects/listobject.c	Mon Nov 18 22:10:39 2013 +0100
@@ -359,14 +359,8 @@ list_repr(PyListObject *v)
 
     _PyUnicodeWriter_Init(&writer);
     writer.overallocate = 1;
-    if (Py_SIZE(v) > 1) {
-        /* "[" + "1" + ", 2" * (len - 1) + "]" */
-        writer.min_length = 1 + 1 + (2 + 1) * (Py_SIZE(v) - 1) + 1;
-    }
-    else {
-        /* "[1]" */
-        writer.min_length = 3;
-    }
+    /* "[" + "1" + ", 2" * (len - 1) + "]" */
+    writer.min_length = 1 + 1 + (2 + 1) * (Py_SIZE(v) - 1) + 1;
 
     if (_PyUnicodeWriter_WriteChar(&writer, '[') < 0)
         goto error;