gh-99249: Clarify "read-only" slots tp_bases & tp_mro (GH-99342)

miss-islington · encukou · kumaraditya303 · web-flow · commit 5dce4ab736e7 · 2022-11-28T03:25:04.000-08:00
These slots are marked "should be treated as read-only" in the table at the start of the document. That doesn't say anything about setting them in the static struct. `tp_bases` docs did say that it should be ``NULL`` (TIL!). If you ignore that, seemingly nothing bad happens. However, some slots may not be inherited, depending on which sub-slot structs are present. (FWIW, NumPy sets tp_bases and is affected by the quirk -- though to be fair, its DUAL_INHERIT code probably predates tp_bases docs, and also the result happens to be benign.) This patch makes things explicit. It also makes the summary table legend easier to scan. (cherry picked from commit 219696a) Co-authored-by: Petr Viktorin <encukou@gmail.com> Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
diff --git a/Doc/c-api/typeobj.rst b/Doc/c-api/typeobj.rst
@@ -149,10 +149,16 @@ Quick Reference
    +------------------------------------------------+-----------------------------------+-------------------+---+---+---+---+
 
 .. [#slots]
-   A slot name in parentheses indicates it is (effectively) deprecated.
-   Names in angle brackets should be treated as read-only.
-   Names in square brackets are for internal use only.
-   "<R>" (as a prefix) means the field is required (must be non-``NULL``).
+
+   **()**: A slot name in parentheses indicates it is (effectively) deprecated.
+
+   **<>**: Names in angle brackets should be initially set to ``NULL`` and
+   treated as read-only.
+
+   **[]**: Names in square brackets are for internal use only.
+
+   **<R>** (as a prefix) means the field is required (must be non-``NULL``).
+
 .. [#cols] Columns:
 
    **"O"**:  set on :c:type:`PyBaseObject_Type`
@@ -1903,8 +1909,19 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
    Tuple of base types.
 
-   This is set for types created by a class statement.  It should be ``NULL`` for
-   statically defined types.
+   This field should be set to ``NULL`` and treated as read-only.
+   Python will fill it in when the type is :c:func:`initialized <PyType_Ready>`.
+
+   For dynamically created classes, the ``Py_tp_bases``
+   :c:type:`slot <PyType_Slot>` can be used instead of the *bases* argument
+   of :c:func:`PyType_FromSpecWithBases`.
+   The argument form is preferred.
+
+   .. warning::
+
+      Multiple inheritance does not work well for statically defined types.
+      If you set ``tp_bases`` to a tuple, Python will not raise an error,
+      but some slots will only be inherited from the first base.
 
    **Inheritance:**
 
@@ -1916,6 +1933,8 @@ and :c:type:`PyType_Type` effectively act as defaults.)
    Tuple containing the expanded set of base types, starting with the type itself
    and ending with :class:`object`, in Method Resolution Order.
 
+   This field should be set to ``NULL`` and treated as read-only.
+   Python will fill it in when the type is :c:func:`initialized <PyType_Ready>`.
 
    **Inheritance:**
 
