Issue #9542: Create PyUnicode_FSDecoder() function

It's a ParseTuple converter: decode bytes objects to unicode using
PyUnicode_DecodeFSDefaultAndSize(); str objects are output as-is.

 * Don't specify surrogateescape error handler in the comments nor the
   documentation, but PyUnicode_DecodeFSDefaultAndSize() and
   PyUnicode_EncodeFSDefault() because these functions use strict error handler
   for the mbcs encoding (on Windows).
 * Remove PyUnicode_FSConverter() comment in unicodeobject.c to avoid
   inconsistency with unicodeobject.h.

Doc/c-api/unicode.rst

@@ -380,13 +380,25 @@ used, passsing :func:`PyUnicode_FSConverter` as the conversion function:
 
 .. cfunction:: int PyUnicode_FSConverter(PyObject* obj, void* result)
 
-   Convert *obj* into *result*, using :cdata:`Py_FileSystemDefaultEncoding`,
-   and the ``"surrogateescape"`` error handler. *result* must be a
-   ``PyObject*``, return a :func:`bytes` object which must be released if it
-   is no longer used.
+   ParseTuple converter: encode :class:`str` objects to :class:`bytes` using
+   :cfunc:`PyUnicode_EncodeFSDefault`; :class:`bytes` objects are output as-is.
+   *result* must be a :ctype:`PyBytesObject*` which must be released when it is
+   no longer used.
 
    .. versionadded:: 3.1
 
+To decode file names during argument parsing, the ``"O&"`` converter should be
+used, passsing :func:`PyUnicode_FSDecoder` as the conversion function:
+
+.. cfunction:: int PyUnicode_FSDecoder(PyObject* obj, void* result)
+
+   ParseTuple converter: decode :class:`bytes` objects to :class:`str` using
+   :cfunc:`PyUnicode_DecodeFSDefaultAndSize`; :class:`str` objects are output
+   as-is. *result* must be a :ctype:`PyUnicodeObject*` which must be released
+   when it is no longer used.
+
+   .. versionadded:: 3.2
+
 .. cfunction:: PyObject* PyUnicode_DecodeFSDefaultAndSize(const char *s, Py_ssize_t size)
 
    Decode a null-terminated string using :cdata:`Py_FileSystemDefaultEncoding`

Include/unicodeobject.h

@@ -200,6 +200,7 @@ typedef PY_UNICODE_TYPE Py_UNICODE;
 # define PyUnicode_FromUnicode PyUnicodeUCS2_FromUnicode
 # define PyUnicode_FromWideChar PyUnicodeUCS2_FromWideChar
 # define PyUnicode_FSConverter PyUnicodeUCS2_FSConverter
+# define PyUnicode_FSDecoder PyUnicodeUCS2_FSDecoder
 # define PyUnicode_GetDefaultEncoding PyUnicodeUCS2_GetDefaultEncoding
 # define PyUnicode_GetMax PyUnicodeUCS2_GetMax
 # define PyUnicode_GetSize PyUnicodeUCS2_GetSize
@@ -300,6 +301,7 @@ typedef PY_UNICODE_TYPE Py_UNICODE;
 # define PyUnicode_FromUnicode PyUnicodeUCS4_FromUnicode
 # define PyUnicode_FromWideChar PyUnicodeUCS4_FromWideChar
 # define PyUnicode_FSConverter PyUnicodeUCS4_FSConverter
+# define PyUnicode_FSDecoder PyUnicodeUCS4_FSDecoder
 # define PyUnicode_GetDefaultEncoding PyUnicodeUCS4_GetDefaultEncoding
 # define PyUnicode_GetMax PyUnicodeUCS4_GetMax
 # define PyUnicode_GetSize PyUnicodeUCS4_GetSize
@@ -1239,12 +1241,16 @@ PyAPI_FUNC(int) PyUnicode_EncodeDecimal(
 
 /* --- File system encoding ---------------------------------------------- */
 
-/* ParseTuple converter which converts a Unicode object into the file
-   system encoding as a bytes object, using the "surrogateescape" error
-   handler; bytes objects are output as-is. */
+/* ParseTuple converter: encode str objects to bytes using
+   PyUnicode_EncodeFSDefault(); bytes objects are output as-is. */
 
 PyAPI_FUNC(int) PyUnicode_FSConverter(PyObject*, void*);
 
+/* ParseTuple converter: decode bytes objects to unicode using
+   PyUnicode_DecodeFSDefaultAndSize(); str objects are output as-is. */
+
+PyAPI_FUNC(int) PyUnicode_FSDecoder(PyObject*, void*);
+
 /* Decode a null-terminated string using Py_FileSystemDefaultEncoding
    and the "surrogateescape" error handler.
 

Misc/NEWS

@@ -12,6 +12,10 @@ What's New in Python 3.2 Alpha 2?
 Core and Builtins
 -----------------
 
+- Issue #9542: Create PyUnicode_FSDecoder() function, a ParseTuple converter:
+  decode bytes objects to unicode using PyUnicode_DecodeFSDefaultAndSize();
+  str objects are output as-is.
+
 - Issue #9203: Computed gotos are now enabled by default on supported
   compilers (which are detected by the configure script).  They can still
   be disable selectively by specifying --without-computed-gotos.

Objects/unicodeobject.c

@@ -1652,9 +1652,6 @@ PyUnicode_DecodeFSDefaultAndSize(const char *s, Py_ssize_t size)
     }
 }
 
-/* Convert the argument to a bytes object, according to the file
-   system encoding.  The addr param must be a PyObject**.
-   This is designed to be used with "O&" in PyArg_Parse APIs. */
 
 int
 PyUnicode_FSConverter(PyObject* arg, void* addr)
@@ -1696,6 +1693,47 @@ PyUnicode_FSConverter(PyObject* arg, void* addr)
 }
 
 
+int
+PyUnicode_FSDecoder(PyObject* arg, void* addr)
+{
+    PyObject *output = NULL;
+    Py_ssize_t size;
+    void *data;
+    if (arg == NULL) {
+        Py_DECREF(*(PyObject**)addr);
+        return 1;
+    }
+    if (PyUnicode_Check(arg)) {
+        output = arg;
+        Py_INCREF(output);
+    }
+    else {
+        arg = PyBytes_FromObject(arg);
+        if (!arg)
+            return 0;
+        output = PyUnicode_DecodeFSDefaultAndSize(PyBytes_AS_STRING(arg),
+                                                  PyBytes_GET_SIZE(arg));
+        Py_DECREF(arg);
+        if (!output)
+            return 0;
+        if (!PyUnicode_Check(output)) {
+            Py_DECREF(output);
+            PyErr_SetString(PyExc_TypeError, "decoder failed to return unicode");
+            return 0;
+        }
+    }
+    size = PyUnicode_GET_SIZE(output);
+    data = PyUnicode_AS_UNICODE(output);
+    if (size != Py_UNICODE_strlen(data)) {
+        PyErr_SetString(PyExc_TypeError, "embedded NUL character");
+        Py_DECREF(output);
+        return 0;
+    }
+    *(PyObject**)addr = output;
+    return Py_CLEANUP_SUPPORTED;
+}
+
+
 char*
 _PyUnicode_AsStringAndSize(PyObject *unicode, Py_ssize_t *psize)
 {