Issue #16147: Rewrite PyUnicode_FromFormatV() to use _PyUnicodeWriter API

 * Simplify the code: replace 4 steps with one unique step using the
   _PyUnicodeWriter API. PyUnicode_Format() has the same design. It avoids to
   store intermediate results which require to allocate an array of pointers on
   the heap.
 * Use the _PyUnicodeWriter API for speed (and its convinient API):
   overallocate the buffer to reduce the number of "realloc()"
 * Implement "width" and "precision" in Python, don't rely on sprintf(). It
   avoids to need of a temporary buffer allocated on the heap: only use a small
   buffer allocated in the stack.
 * Add _PyUnicodeWriter_WriteCstr() function
 * Split PyUnicode_FromFormatV() into two functions: add
   unicode_fromformat_arg().
 * Inline parse_format_flags(): the format of an argument is now only parsed
   once, it's no more needed to have a subfunction.
 * Optimize PyUnicode_FromFormatV() for characters between two "%" arguments:
   search the next "%" and copy the substring in one chunk, instead of copying
   character per character.

Include/unicodeobject.h

@@ -933,12 +933,28 @@ PyAPI_FUNC(int)
 _PyUnicodeWriter_PrepareInternal(_PyUnicodeWriter *writer,
                                  Py_ssize_t length, Py_UCS4 maxchar);
 
+/* Append a Unicode string.
+   Return 0 on success, raise an exception and return -1 on error. */
 PyAPI_FUNC(int)
-_PyUnicodeWriter_WriteStr(_PyUnicodeWriter *writer, PyObject *str);
+_PyUnicodeWriter_WriteStr(_PyUnicodeWriter *writer,
+    PyObject *str               /* Unicode string */
+    );
 
+/* Append a latin1-encoded byte string.
+   Return 0 on success, raise an exception and return -1 on error. */
+PyAPI_FUNC(int)
+_PyUnicodeWriter_WriteCstr(_PyUnicodeWriter *writer,
+    const char *str,            /* latin1-encoded byte string */
+    Py_ssize_t len              /* length in bytes */
+    );
+
+/* Get the value of the write as an Unicode string. Clear the
+   buffer of the writer. Raise an exception and return NULL
+   on error. */
 PyAPI_FUNC(PyObject *)
 _PyUnicodeWriter_Finish(_PyUnicodeWriter *writer);
 
+/* Deallocate memory of a writer (clear its internal buffer). */
 PyAPI_FUNC(void)
 _PyUnicodeWriter_Dealloc(_PyUnicodeWriter *writer);
 #endif

Lib/test/test_unicode.py

@@ -1769,6 +1769,22 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertEqual(PyUnicode_FromFormat(b'%llu', c_ulonglong(123)), '123')
         self.assertEqual(PyUnicode_FromFormat(b'%zu', c_size_t(123)), '123')
 
+        # test padding (width and/or precision)
+        self.assertEqual(PyUnicode_FromFormat(b'%010i', c_int(123)), '123'.rjust(10, '0'))
+        self.assertEqual(PyUnicode_FromFormat(b'%100i', c_int(123)), '123'.rjust(100))
+        self.assertEqual(PyUnicode_FromFormat(b'%.100i', c_int(123)), '123'.rjust(100, '0'))
+        self.assertEqual(PyUnicode_FromFormat(b'%100.80i', c_int(123)), '123'.rjust(80, '0').rjust(100))
+
+        self.assertEqual(PyUnicode_FromFormat(b'%010u', c_uint(123)), '123'.rjust(10, '0'))
+        self.assertEqual(PyUnicode_FromFormat(b'%100u', c_uint(123)), '123'.rjust(100))
+        self.assertEqual(PyUnicode_FromFormat(b'%.100u', c_uint(123)), '123'.rjust(100, '0'))
+        self.assertEqual(PyUnicode_FromFormat(b'%100.80u', c_uint(123)), '123'.rjust(80, '0').rjust(100))
+
+        self.assertEqual(PyUnicode_FromFormat(b'%010x', c_int(0x123)), '123'.rjust(10, '0'))
+        self.assertEqual(PyUnicode_FromFormat(b'%100x', c_int(0x123)), '123'.rjust(100))
+        self.assertEqual(PyUnicode_FromFormat(b'%.100x', c_int(0x123)), '123'.rjust(100, '0'))
+        self.assertEqual(PyUnicode_FromFormat(b'%100.80x', c_int(0x123)), '123'.rjust(80, '0').rjust(100))
+
         # test %A
         text = PyUnicode_FromFormat(b'%%A:%A', 'abc\xe9\uabcd\U0010ffff')
         self.assertEqual(text, r"%A:'abc\xe9\uabcd\U0010ffff'")