Add _PyUnicodeWriter_WriteSubstring() function

Write a function to enable more optimizations:

 * If the substring is the whole string and overallocation is disabled, just
   keep a reference to the string, don't copy characters
 * Avoid a call to the expensive _PyUnicode_FindMaxChar() function when
   possible

Include/unicodeobject.h

@@ -940,6 +940,15 @@ _PyUnicodeWriter_WriteStr(_PyUnicodeWriter *writer,
     PyObject *str               /* Unicode string */
     );
 
+/* Append a substring of a Unicode string.
+   Return 0 on success, raise an exception and return -1 on error. */
+PyAPI_FUNC(int)
+_PyUnicodeWriter_WriteSubstring(_PyUnicodeWriter *writer,
+    PyObject *str,              /* Unicode string */
+    Py_ssize_t start,
+    Py_ssize_t end
+    );
+
 /* Append a latin1-encoded byte string.
    Return 0 on success, raise an exception and return -1 on error. */
 PyAPI_FUNC(int)

Lib/test/test_format.py

@@ -324,6 +324,9 @@ class FormatTest(unittest.TestCase):
         self.assertIs("{0:1s}".format(text), text)
         self.assertIs("{0:5s}".format(text), text)
 
+        self.assertIs(text % (), text)
+        self.assertIs(text.format(), text)
+
 
 def test_main():
     support.run_unittest(FormatTest)

Objects/stringlib/unicode_format.h

@@ -869,25 +869,19 @@ do_markup(SubString *input, PyObject *args, PyObject *kwargs,
     SubString literal;
     SubString field_name;
     SubString format_spec;
-    Py_UCS4 conversion, maxchar;
-    Py_ssize_t sublen;
-    int err;
+    Py_UCS4 conversion;
 
     MarkupIterator_init(&iter, input->str, input->start, input->end);
     while ((result = MarkupIterator_next(&iter, &literal, &field_present,
                                          &field_name, &format_spec,
                                          &conversion,
                                          &format_spec_needs_expanding)) == 2) {
-        sublen = literal.end - literal.start;
-        if (sublen) {
-            maxchar = _PyUnicode_FindMaxChar(literal.str,
-                                             literal.start, literal.end);
-            err = _PyUnicodeWriter_Prepare(writer, sublen, maxchar);
-            if (err == -1)
+        if (literal.end != literal.start) {
+            if (!field_present && iter.str.start == iter.str.end)
+                writer->overallocate = 0;
+            if (_PyUnicodeWriter_WriteSubstring(writer, literal.str,
+                                                literal.start, literal.end) < 0)
                 return 0;
-            _PyUnicode_FastCopyCharacters(writer->buffer, writer->pos,
-                                          literal.str, literal.start, sublen);
-            writer->pos += sublen;
         }
 
         if (field_present) {

Objects/unicodeobject.c

@@ -12790,6 +12790,41 @@ _PyUnicodeWriter_WriteStr(_PyUnicodeWriter *writer, PyObject *str)
     return 0;
 }
 
+int
+_PyUnicodeWriter_WriteSubstring(_PyUnicodeWriter *writer, PyObject *str,
+                                Py_ssize_t start, Py_ssize_t end)
+{
+    Py_UCS4 maxchar;
+    Py_ssize_t len;
+
+    if (PyUnicode_READY(str) == -1)
+        return -1;
+
+    assert(0 <= start);
+    assert(end <= PyUnicode_GET_LENGTH(str));
+    assert(start <= end);
+
+    if (end == 0)
+        return 0;
+
+    if (start == 0 && end == PyUnicode_GET_LENGTH(str))
+        return _PyUnicodeWriter_WriteStr(writer, str);
+
+    if (PyUnicode_MAX_CHAR_VALUE(str) > writer->maxchar)
+        maxchar = _PyUnicode_FindMaxChar(str, start, end);
+    else
+        maxchar = writer->maxchar;
+    len = end - start;
+
+    if (_PyUnicodeWriter_Prepare(writer, len, maxchar) < 0)
+        return -1;
+
+    _PyUnicode_FastCopyCharacters(writer->buffer, writer->pos,
+                                  str, start, len);
+    writer->pos += len;
+    return 0;
+}
+
 int
 _PyUnicodeWriter_WriteCstr(_PyUnicodeWriter *writer, const char *str, Py_ssize_t len)
 {
@@ -13963,7 +13998,7 @@ PyUnicode_Format(PyObject *format, PyObject *args)
 
     while (--ctx.fmtcnt >= 0) {
         if (PyUnicode_READ(ctx.fmtkind, ctx.fmtdata, ctx.fmtpos) != '%') {
-            Py_ssize_t nonfmtpos, sublen;
+            Py_ssize_t nonfmtpos;
             Py_UCS4 maxchar;
 
             nonfmtpos = ctx.fmtpos++;
@@ -13976,15 +14011,10 @@ PyUnicode_Format(PyObject *format, PyObject *args)
                 ctx.fmtpos--;
                 ctx.writer.overallocate = 0;
             }
-            sublen = ctx.fmtpos - nonfmtpos;
-            maxchar = _PyUnicode_FindMaxChar(ctx.fmtstr,
-                                             nonfmtpos, nonfmtpos + sublen);
-            if (_PyUnicodeWriter_Prepare(&ctx.writer, sublen, maxchar) == -1)
-                goto onError;
 
-            _PyUnicode_FastCopyCharacters(ctx.writer.buffer, ctx.writer.pos,
-                                          ctx.fmtstr, nonfmtpos, sublen);
-            ctx.writer.pos += sublen;
+            if (_PyUnicodeWriter_WriteSubstring(&ctx.writer, ctx.fmtstr,
+                                                nonfmtpos, ctx.fmtpos) < 0)
+                goto onError;
         }
         else {
             ctx.fmtpos++;