pythongh-91417: Remove PySequence_Fast() from the limited C API (python#129398)

The function never worked with the limited C API. It was added by
mistake.

Author: vstinner

Doc/data/stable_abi.dat

@@ -1343,6 +1343,11 @@ Limited C API changes
   implementation details.
   (Contributed by Victor Stinner in :gh:`120600` and :gh:`124127`.)
 
+* Remove :c:func:`PySequence_Fast` from the limited C API, since this function
+  has to be used with :c:macro:`PySequence_Fast_GET_ITEM` which never worked
+  in the limited C API.
+  (Contributed by Victor Stinner in :gh:`91417`.)
+
 
 Porting to Python 3.14
 ----------------------

Doc/whatsnew/3.14.rst

@@ -726,31 +726,6 @@ PyAPI_FUNC(PyObject *) PySequence_Tuple(PyObject *o);
    This is equivalent to the Python expression: list(o) */
 PyAPI_FUNC(PyObject *) PySequence_List(PyObject *o);
 
-/* Return the sequence 'o' as a list, unless it's already a tuple or list.
-
-   Use PySequence_Fast_GET_ITEM to access the members of this list, and
-   PySequence_Fast_GET_SIZE to get its length.
-
-   Returns NULL on failure.  If the object does not support iteration, raises a
-   TypeError exception with 'm' as the message text. */
-PyAPI_FUNC(PyObject *) PySequence_Fast(PyObject *o, const char* m);
-
-/* Return the size of the sequence 'o', assuming that 'o' was returned by
-   PySequence_Fast and is not NULL. */
-#define PySequence_Fast_GET_SIZE(o) \
-    (PyList_Check(o) ? PyList_GET_SIZE(o) : PyTuple_GET_SIZE(o))
-
-/* Return the 'i'-th element of the sequence 'o', assuming that o was returned
-   by PySequence_Fast, and that i is within bounds. */
-#define PySequence_Fast_GET_ITEM(o, i)\
-     (PyList_Check(o) ? PyList_GET_ITEM((o), (i)) : PyTuple_GET_ITEM((o), (i)))
-
-/* Return a pointer to the underlying item array for
-   an object returned by PySequence_Fast */
-#define PySequence_Fast_ITEMS(sf) \
-    (PyList_Check(sf) ? ((PyListObject *)(sf))->ob_item \
-                      : ((PyTupleObject *)(sf))->ob_item)
-
 /* Return the number of occurrences on value on 'o', that is, return
    the number of keys for which o[key] == value.
 

Include/abstract.h

@@ -85,3 +85,29 @@ PyAPI_FUNC(Py_ssize_t) PyObject_LengthHint(PyObject *o, Py_ssize_t);
    need to be corrected for a negative index. */
 #define PySequence_ITEM(o, i)\
     ( Py_TYPE(o)->tp_as_sequence->sq_item((o), (i)) )
+
+/* Return the sequence 'o' as a list, unless it's already a tuple or list.
+
+   Use PySequence_Fast_GET_ITEM to access the members of this list, and
+   PySequence_Fast_GET_SIZE to get its length.
+
+   Returns NULL on failure.  If the object does not support iteration, raises a
+   TypeError exception with 'm' as the message text. */
+PyAPI_FUNC(PyObject *) PySequence_Fast(PyObject *o, const char* m);
+
+/* Return the size of the sequence 'o', assuming that 'o' was returned by
+   PySequence_Fast and is not NULL. */
+#define PySequence_Fast_GET_SIZE(o) \
+    (PyList_Check(o) ? PyList_GET_SIZE(o) : PyTuple_GET_SIZE(o))
+
+/* Return the 'i'-th element of the sequence 'o', assuming that o was returned
+   by PySequence_Fast, and that i is within bounds. */
+#define PySequence_Fast_GET_ITEM(o, i)\
+     (PyList_Check(o) ? PyList_GET_ITEM((o), (i)) : PyTuple_GET_ITEM((o), (i)))
+
+/* Return a pointer to the underlying item array for
+   an object returned by PySequence_Fast */
+#define PySequence_Fast_ITEMS(sf) \
+    (PyList_Check(sf) ? ((PyListObject *)(sf))->ob_item \
+                      : ((PyTupleObject *)(sf))->ob_item)
+

Include/cpython/abstract.h

@@ -0,0 +1,3 @@
+Remove :c:func:`PySequence_Fast` from the limited C API, since this function
+has to be used with :c:macro:`PySequence_Fast_GET_ITEM` which never worked
+in the limited C API. Patch by Victor Stinner.

Misc/NEWS.d/next/C_API/2025-01-28-13-21-17.gh-issue-91417.AfiR0t.rst

@@ -1253,6 +1253,7 @@
     added = '3.2'
 [function.PySequence_Fast]
     added = '3.2'
+    abi_only = true
 [function.PySequence_GetItem]
     added = '3.2'
 [function.PySequence_GetSlice]