Add test and comments

Author: vstinner

Lib/test/test_capi/test_unicode.py

@@ -1709,6 +1709,10 @@ def test_unicode_export(self):
                          ('ucs4:\U0010ffff'.encode(ucs4_enc),
                           PyUnicode_FORMAT_UCS4))
 
+        # export ASCII as UCS1
+        self.assertEqual(unicode_export("abc", PyUnicode_FORMAT_UCS1),
+                         (b'abc', PyUnicode_FORMAT_UCS1))
+
         # always export to UCS4
         self.assertEqual(unicode_export("abc", PyUnicode_FORMAT_UCS4),
                          ('abc'.encode(ucs4_enc), PyUnicode_FORMAT_UCS4))

Objects/unicodeobject.c

@@ -2142,6 +2142,7 @@ PyUnicode_Export(PyObject *unicode, unsigned int supported_formats,
     }
 
     if (supported_formats & PyUnicode_FORMAT_UCS4) {
+        // Convert UCS1 or UCS2 to UCS4
         Py_UCS4 *ucs4 = PyUnicode_AsUCS4Copy(unicode);
         if (ucs4 == NULL) {
             goto error;
@@ -2152,6 +2153,7 @@ PyUnicode_Export(PyObject *unicode, unsigned int supported_formats,
     }
 
     if (supported_formats & PyUnicode_FORMAT_UTF8) {
+        // Encode UCS1, UCS2 or UCS4 to UTF-8
         const char *utf8 = PyUnicode_AsUTF8AndSize(unicode, size);
         if (utf8 == NULL) {
             goto error;