bpo-40596: Fix str.isidentifier() for non-canonicalized strings containing non-BMP characters on Windows. (GH-20053)

Lib/test/test_unicode.py

@@ -720,6 +720,13 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertFalse("©".isidentifier())
         self.assertFalse("0".isidentifier())
 
+    @support.cpython_only
+    def test_isidentifier_legacy(self):
+        import _testcapi
+        u = '𝖀𝖓𝖎𝖈𝖔𝖉𝖊'
+        self.assertTrue(u.isidentifier())
+        self.assertTrue(_testcapi.unicode_legacy_string(u).isidentifier())
+
     def test_isprintable(self):
         self.assertTrue("".isprintable())
         self.assertTrue(" ".isprintable())

Misc/NEWS.d/next/Core and Builtins/2020-05-11-20-53-52.bpo-40596.dwOH_X.rst

@@ -0,0 +1,2 @@
+Fixed :meth:`str.isidentifier` for non-canonicalized strings containing
+non-BMP characters on Windows.

Objects/unicodeobject.c

@@ -12356,20 +12356,38 @@ PyUnicode_IsIdentifier(PyObject *self)
         return len && i == len;
     }
     else {
-        Py_ssize_t i, len = PyUnicode_GET_SIZE(self);
+        Py_ssize_t i = 0, len = PyUnicode_GET_SIZE(self);
         if (len == 0) {
             /* an empty string is not a valid identifier */
             return 0;
         }
 
         const wchar_t *wstr = _PyUnicode_WSTR(self);
-        Py_UCS4 ch = wstr[0];
+        Py_UCS4 ch = wstr[i++];
+#if SIZEOF_WCHAR_T == 2
+        if (Py_UNICODE_IS_HIGH_SURROGATE(ch)
+            && i < len
+            && Py_UNICODE_IS_LOW_SURROGATE(wstr[i]))
+        {
+            ch = Py_UNICODE_JOIN_SURROGATES(ch, wstr[i]);
+            i++;
+        }
+#endif
         if (!_PyUnicode_IsXidStart(ch) && ch != 0x5F /* LOW LINE */) {
             return 0;
         }
 
-        for (i = 1; i < len; i++) {
-            ch = wstr[i];
+        while (i < len) {
+            ch = wstr[i++];
+#if SIZEOF_WCHAR_T == 2
+            if (Py_UNICODE_IS_HIGH_SURROGATE(ch)
+                && i < len
+                && Py_UNICODE_IS_LOW_SURROGATE(wstr[i]))
+            {
+                ch = Py_UNICODE_JOIN_SURROGATES(ch, wstr[i]);
+                i++;
+            }
+#endif
             if (!_PyUnicode_IsXidContinue(ch)) {
                 return 0;
             }