PEP 820: Remove PySlot_HAS_FALLBACK (GH-4861)

Unlike the rest of the PEP, this can be added in a later version,
when needed.

Author: encukou

peps/pep-0820.rst

@@ -122,29 +122,21 @@ The macros simplify hand-written literals.
 For more complex use cases, like compatibility between several Python versions,
 or templated/auto-generated slot arrays, as well as for non-C users of the
 C API, the slot struct definitions can be written out.
-For example, if the transition from ``tp_getattr`` to ``tp_getattro``
-was happening in the near future (say, CPython 3.17), rather than 1.4, and
-the user wanted to support CPython with and without ``tp_getattro``, they could
-add a "``HAS_FALLBACK``" flag::
+For example, if the ``nb_matrix_multiply`` slot (:pep:`465`) was added in the
+near future (say, CPython 3.17) rather than in 3.5, users could add it with
+an "``OPTIONAL``" flag, making their class support the ``@`` operator only
+on CPython versions with that operator::
 
     static PySlot myClass_slots[] = {
        ...
        {   // skipped if not supported
-           .sl_id=Py_tp_getattro,
-           .sl_flags=PySlot_HAS_FALLBACK,
-           .sl_func=myClass_getattro,
-       },
-       {    // used if if the slot above was skipped
-           .sl_id=Py_tp_getattr,
-           .sl_func=myClass_old_getattr,
+           .sl_id=Py_nb_matrix_multiply,
+           .sl_flags=PySlot_OPTIONAL,
+           .sl_func=myClass_matmul,
        },
        PySlot_END,
     }
 
-Similarly, if the ``nb_matrix_multiply`` slot (:pep:`465`) was added in the
-near future, users could add it with an "``OPTIONAL``" flag, making their class
-support the ``@`` operator only on CPython versions with that operator.
-
 
 .. _pep820-rationale:
 
@@ -400,17 +392,6 @@ Flags
   *array* of ``PyMemberDef`` structures, then the entire array, as well as the
   ``name`` and ``doc`` strings in its elements, must be static and constant.
 
-- ``PySlot_HAS_FALLBACK``: If the slot ID is unknown, the interpreter will
-  ignore the slot.
-  If it's known, the interpreter should ignore subsequent slots up to
-  (and including) the first one without HAS_FALLBACK.
-
-  Effectively, consecutive slots with the HAS_FALLBACK flag, plus the first
-  non-HAS_FALLBACK slot after them, form a “block” where the the interpreter
-  will only consider the *first* slot in the block that it understands.
-  If the entire block is to be optional, it should end with a
-  slot with the OPTIONAL flag.
-
 - ``PySlot_INTPTR``: The data is stored in ``sl_ptr``, and must be cast to
   the appropriate type.
 
@@ -474,13 +455,8 @@ Each ``PyType_Slot`` in the array will be converted to
 ``(PySlot){.sl_id=slot, .sl_flags=PySlot_INTPTR, .sl_ptr=func}``,
 and similar with ``PyModuleDef_Slot``.
 
-The initial implementation will have restrictions that may be lifted
-in the future:
-
-- ``Py_slot_subslots``, ``Py_tp_slots`` and ``Py_mod_slots`` cannot use
-  ``PySlot_HAS_FALLBACK`` (the flag cannot be set on them nor a slot that
-  precedes them).
-- Nesting depth will be limited to 5 levels.
+In the initial implementation, nesting depth will be limited to 5 levels.
+This restrictions may be lifted in the future.
 
 
 New slot IDs
@@ -492,8 +468,7 @@ definitions, will be added:
 - ``Py_slot_end`` (defined as ``0``): Marks the end of a slots array.
 
   - The ``PySlot_INTPTR`` and ``PySlot_STATIC`` flags are ignored.
-  - The ``PySlot_OPTIONAL`` and ``PySlot_HAS_FALLBACK`` flags are not
-    allowed with ``Py_slot_end``.
+  - The ``PySlot_OPTIONAL`` flags is not allowed with ``Py_slot_end``.
 
 - ``Py_slot_subslots``, ``Py_tp_slots``, ``Py_mod_slots``: see
   :ref:`pep820-nested-tables` above
@@ -701,6 +676,25 @@ For a bigger picture: anonymous unions can be a helpful tool for implemeting
 tagged unions and for evolving public API in backwards-compatible ways.
 This PEP intentionally opens the door to using them more often.
 
+Fallback slots
+--------------
+
+An earlier version of this PEP proposed a flag: ``PySlot_HAS_FALLBACK``:
+
+  If the flagged slot's ID is unknown, the interpreter will ignore the slot.
+  If it's known, the interpreter should ignore subsequent slots up to
+  (and including) the first one without HAS_FALLBACK.
+
+  Effectively, consecutive slots with the HAS_FALLBACK flag, plus the first
+  non-HAS_FALLBACK slot after them, form a "block" where the the interpreter
+  will only consider the *first* slot in the block that it understands.
+  If the entire block is to be optional, it should end with a
+  slot with the OPTIONAL flag.
+
+This flag may be added later, in the Python version where it's needed.
+(For backwards compatibility, all slots flagged HAS_FALLBACK will also need
+the OPTIONAL flag).
+
 
 Open Issues
 ===========
@@ -711,10 +705,35 @@ None yet.
 Acknowledgements
 ================
 
-Thanks to Da Woods, Antoine Pitrou and Mark Shannon
+Thanks to Da Woods, Antoine Pitrou, Mark Shannon and Victor Stinner
 for substantial input on this iteration of the proposal.
 
 
+Change History
+==============
+
+* `12-Mar-2026 <https://discuss.python.org/t/105552/12>`__
+  - Remove unnecessary flag ``PySlot_HAS_FALLBACK``
+
+* `28-Jan-2026 <https://discuss.python.org/t/105552/6>`__
+
+  - Be clearer that the PEP 793 API added in 3.15 alpha (``PyModExport``,
+    ``PyModule_FromSlotsAndSpec``) will be changed to return the new slots.
+
+  - Deprecation of things that were documented to not work, but
+    worked in practice:
+
+    - setting a slot value to NULL (except if the slot explicitly allows this)
+    - repeating a slot ID in a definition (except )
+
+  - Add "third-party slot ID allocation" and "avoiding anonymous unions"
+    to Rejected Ideas.
+
+
+* `06-Jan-2025 <https://discuss.python.org/t/105552>`__
+  - Initial PEP
+
+
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