diff -r 99ef4501205b Objects/stringlib/unicode_format.h
--- a/Objects/stringlib/unicode_format.h	Thu May 10 16:11:12 2012 +0100
+++ b/Objects/stringlib/unicode_format.h	Sun May 13 04:18:09 2012 +0200
@@ -902,11 +902,14 @@ do_markup(SubString *input, PyObject *ar
             writer->pos += sublen;
         }
 
-        if (field_present)
+        if (field_present) {
+            if (iter.str.start == iter.str.end)
+                writer->overallocate = 0;
             if (!output_markup(&field_name, &format_spec,
                                format_spec_needs_expanding, conversion, writer,
                                args, kwargs, recursion_depth, auto_number))
                 return 0;
+        }
     }
     return result;
 }
@@ -921,7 +924,6 @@ build_string(SubString *input, PyObject 
              int recursion_depth, AutoNumber *auto_number)
 {
     _PyUnicodeWriter writer;
-    Py_ssize_t initlen;
 
     /* check the recursion level */
     if (recursion_depth <= 0) {
@@ -930,8 +932,7 @@ build_string(SubString *input, PyObject 
         return NULL;
     }
 
-    initlen = PyUnicode_GET_LENGTH(input->str) + 100;
-    if (_PyUnicodeWriter_Init(&writer, initlen, 127) == -1)
+    if (_PyUnicodeWriter_Init(&writer, 0, 0) == -1)
         return NULL;
 
     if (!do_markup(input, args, kwargs, &writer, recursion_depth,
diff -r 99ef4501205b Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Thu May 10 16:11:12 2012 +0100
+++ b/Objects/unicodeobject.c	Sun May 13 04:18:09 2012 +0200
@@ -12893,6 +12893,7 @@ unicode_endswith(PyObject *self,
 }
 
 typedef struct {
+    int overallocate;
     PyObject *buffer;
     void *data;
     enum PyUnicode_Kind kind;
@@ -12910,13 +12911,23 @@ _PyUnicodeWriter_Update(_PyUnicodeWriter
 
 Py_LOCAL(int)
 _PyUnicodeWriter_Init(_PyUnicodeWriter *writer,
-                    Py_ssize_t length, Py_UCS4 maxchar)
-{
+                      Py_ssize_t length, Py_UCS4 maxchar)
+{
+    writer->overallocate = 1;
     writer->pos = 0;
-    writer->buffer = PyUnicode_New(length, maxchar);
-    if (writer->buffer == NULL)
-        return -1;
-    _PyUnicodeWriter_Update(writer);
+    if (length != 0) {
+        writer->buffer = PyUnicode_New(length, maxchar);
+        if (writer->buffer == NULL)
+            return -1;
+        _PyUnicodeWriter_Update(writer);
+    }
+    else {
+        writer->buffer = NULL;
+        writer->maxchar = 0;
+        writer->data = NULL;
+        /* invalid kind */
+        writer->kind = 5;
+    }
     return 0;
 }
 
@@ -12933,10 +12944,29 @@ _PyUnicodeWriter_Prepare(_PyUnicodeWrite
     }
     newlen = writer->pos + length;
 
+    if (writer->buffer == NULL) {
+        if (writer->overallocate) {
+            /* overallocate by 100% to limit the number of resize */
+            if (newlen <= PY_SSIZE_T_MAX / 2)
+                newlen *= 2;
+            if (newlen < 100)
+                newlen = 100;
+        }
+        writer->buffer = PyUnicode_New(newlen, maxchar);
+        if (writer->buffer == NULL)
+            return -1;
+        _PyUnicodeWriter_Update(writer);
+        return 0;
+    }
+
     if (newlen > PyUnicode_GET_LENGTH(writer->buffer)) {
-        /* overallocate 25% to limit the number of resize */
-        if (newlen <= (PY_SSIZE_T_MAX - newlen / 4))
-            newlen += newlen / 4;
+        if (writer->overallocate) {
+            /* overallocate by 100% to limit the number of resize */
+            if (newlen <= PY_SSIZE_T_MAX / 2)
+                newlen *= 2;
+            if (newlen < 100)
+                newlen = 100;
+        }
 
         if (maxchar > writer->maxchar) {
             /* resize + widen */
@@ -12966,9 +12996,19 @@ _PyUnicodeWriter_Prepare(_PyUnicodeWrite
 Py_LOCAL(PyObject *)
 _PyUnicodeWriter_Finish(_PyUnicodeWriter *writer)
 {
-    if (PyUnicode_Resize(&writer->buffer, writer->pos) < 0) {
-        Py_DECREF(writer->buffer);
-        return NULL;
+    if (writer->pos == 0) {
+        Py_XDECREF(writer->buffer);
+        Py_INCREF(unicode_empty);
+        return unicode_empty;
+    }
+    if (PyUnicode_GET_LENGTH(writer->buffer) != writer->pos) {
+        PyObject *newbuffer;
+        newbuffer = resize_compact(writer->buffer, writer->pos);
+        if (newbuffer == NULL) {
+            Py_DECREF(writer->buffer);
+            return NULL;
+        }
+        writer->buffer = newbuffer;
     }
     assert(_PyUnicode_CheckConsistency(writer->buffer, 1));
     return writer->buffer;
@@ -13459,7 +13499,7 @@ PyUnicode_Format(PyObject *format, PyObj
     fmtcnt = PyUnicode_GET_LENGTH(uformat);
     fmtpos = 0;
 
-    if (_PyUnicodeWriter_Init(&writer, fmtcnt + 100, 127) < 0)
+    if (_PyUnicodeWriter_Init(&writer, 0, 0) < 0)
         goto onError;
 
     if (PyTuple_Check(args)) {
@@ -13810,6 +13850,9 @@ PyUnicode_Format(PyObject *format, PyObj
             if (sign && len == width)
                 buflen++;
 
+            if (fmtcnt < 1)
+                writer.overallocate = 0;
+
             if (_PyUnicodeWriter_Prepare(&writer, buflen, bufmaxchar) == -1)
                 goto onError;