python · vstinner · Jan 30, 2020 · Jan 30, 2020 · Jan 30, 2020 · Jan 30, 2020
diff --git a/Doc/c-api/unicode.rst b/Doc/c-api/unicode.rst
@@ -239,6 +239,17 @@ access internal read-only data of Unicode objects:
       Part of the old-style Unicode API, please migrate to using the
       :c:func:`PyUnicode_nBYTE_DATA` family of macros.
 
+.. c:function:: int PyUnicode_IsIdentifier(PyObject *o)
+
+   Return ``1`` if the string is a valid identifier according to the language
+   definition, section :ref:`identifiers`. Return ``0`` otherwise.
+
+   Raise an exception and return ``-1`` on error.
+
+   .. versionchanged:: 3.9
+      The function now returns ``-1`` on error, instead of calling
+      :c:func:`Py_FatalError`.
+
 
 Unicode Character Properties
 """"""""""""""""""""""""""""

diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
@@ -339,6 +339,10 @@ Build and C API Changes
   functions are now required to build Python.
   (Contributed by Victor Stinner in :issue:`39395`.)
 
+* The :c:func:`PyUnicode_IsIdentifier` function now returns ``-1`` on error,
+  instead of calling :c:func:`Py_FatalError`.
+  (Contributed by Victor Stinner in :issue:`39500`.)
+
 
 Deprecated
 ==========

diff --git a/Objects/typeobject.c b/Objects/typeobject.c
@@ -2241,7 +2241,11 @@ valid_identifier(PyObject *s)
                      Py_TYPE(s)->tp_name);
         return 0;
     }
-    if (!PyUnicode_IsIdentifier(s)) {
+    int identifier = PyUnicode_IsIdentifier(s);
+    if (identifier < 0) {
+        return 0;
+    }
+    if (!identifier) {
         PyErr_SetString(PyExc_TypeError,
                         "__slots__ must be identifiers");
         return 0;

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
@@ -12209,13 +12209,13 @@ PyUnicode_IsIdentifier(PyObject *self)
     Py_UCS4 first;
 
     if (PyUnicode_READY(self) == -1) {
-        Py_FatalError("identifier not ready");
-        return 0;
+        return -1;
     }
 
-    /* Special case for empty strings */
-    if (PyUnicode_GET_LENGTH(self) == 0)
+    if (PyUnicode_GET_LENGTH(self) == 0) {
+        /* an empty string is not a valid identifier */
         return 0;
+    }
     kind = PyUnicode_KIND(self);
     data = PyUnicode_DATA(self);
 
@@ -12228,12 +12228,15 @@ PyUnicode_IsIdentifier(PyObject *self)
        to check just for these, except that _ must be allowed
        as starting an identifier.  */
     first = PyUnicode_READ(kind, data, 0);
-    if (!_PyUnicode_IsXidStart(first) && first != 0x5F /* LOW LINE */)
+    if (!_PyUnicode_IsXidStart(first) && first != 0x5F /* LOW LINE */) {
         return 0;
+    }
 
-    for (i = 1; i < PyUnicode_GET_LENGTH(self); i++)
-        if (!_PyUnicode_IsXidContinue(PyUnicode_READ(kind, data, i)))
+    for (i = 1; i < PyUnicode_GET_LENGTH(self); i++) {
+        if (!_PyUnicode_IsXidContinue(PyUnicode_READ(kind, data, i))) {
             return 0;
+        }
+    }
     return 1;
 }
 
@@ -12250,7 +12253,11 @@ static PyObject *
 unicode_isidentifier_impl(PyObject *self)
 /*[clinic end generated code: output=fe585a9666572905 input=2d807a104f21c0c5]*/
 {
-    return PyBool_FromLong(PyUnicode_IsIdentifier(self));
+    int identifier = PyUnicode_IsIdentifier(self);
+    if (identifier < 0) {
+        return NULL;
+    }
+    return PyBool_FromLong(identifier);
 }
 
 /*[clinic input]

diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
@@ -1078,6 +1078,11 @@ verify_identifier(struct tok_state *tok)
         return 0;
     }
     result = PyUnicode_IsIdentifier(s);
+    assert(result >= 0);
+    if (result < 0) {
+        /* silently ignore error */
+        PyErr_Clear();
+    }
     Py_DECREF(s);
     if (result == 0)
         tok->done = E_IDENTIFIER;