gh-75459: Doc: C API: Improve object life cycle documentation

.gitattributes

@@ -10,6 +10,7 @@
 *.ico binary
 *.jpg binary
 *.pck binary
+*.pdf binary
 *.png binary
 *.psd binary
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@@ -67,6 +68,7 @@ PCbuild/readme.txt  dos
 **/clinic/*.cpp.h                                   generated
 **/clinic/*.h.h                                     generated
 *_db.h                                              generated
+Doc/c-api/lifecycle.dot.svg                         generated
 Doc/data/stable_abi.dat                             generated
 Doc/library/token-list.inc                          generated
 Include/internal/pycore_ast.h                       generated

Doc/c-api/allocation.rst

@@ -16,46 +16,126 @@ Allocating Objects on the Heap
 
    Initialize a newly allocated object *op* with its type and initial
    reference.  Returns the initialized object.  Other fields of the object are
-   not affected.
+   not initialized.  Despite its name, this function is unrelated to the
+   object's :meth:`~object.__init__` method (:c:member:`~PyTypeObject.tp_init`
+   slot).  Specifically, this function does **not** call the object's
+   :meth:`!__init__` method.
+
+   In general, consider this function to be a low-level routine. Use
+   :c:member:`~PyTypeObject.tp_alloc` where possible.
+
+   .. note::
+
+      This function only initializes the object's memory corresponding to the
+      initial :c:type:`PyObject` structure.  It does not zero the rest.
 
 
 .. c:function:: PyVarObject* PyObject_InitVar(PyVarObject *op, PyTypeObject *type, Py_ssize_t size)
 
    This does everything :c:func:`PyObject_Init` does, and also initializes the
    length information for a variable-size object.
 
+   .. warning::
+
+      This function only initializes some of the object's memory.  It does not
+      zero the rest.
+
 
 .. c:macro:: PyObject_New(TYPE, typeobj)
 
-   Allocate a new Python object using the C structure type *TYPE*
-   and the Python type object *typeobj* (``PyTypeObject*``).
-   Fields not defined by the Python object header are not initialized.
-   The caller will own the only reference to the object
-   (i.e. its reference count will be one).
-   The size of the memory allocation is determined from the
-   :c:member:`~PyTypeObject.tp_basicsize` field of the type object.
+   Allocates a new Python object using the C structure type *TYPE* and the
+   Python type object *typeobj* (``PyTypeObject*``) by calling
+   :c:func:`PyObject_Malloc` to allocate memory and initializing it like
+   :c:func:`PyObject_Init`.  The caller will own the only reference to the
+   object (i.e. its reference count will be one).
+
+   Avoid calling this directly to allocate memory for an object; call the type's
+   :c:member:`~PyTypeObject.tp_alloc` slot instead.
+
+   When populating a type's :c:member:`~PyTypeObject.tp_alloc` slot,
+   :c:func:`PyType_GenericAlloc` is preferred over a custom function that
+   simply calls this macro.
+
+   This macro does not call :c:member:`~PyTypeObject.tp_alloc`,
+   :c:member:`~PyTypeObject.tp_new` (:meth:`~object.__new__`), or
+   :c:member:`~PyTypeObject.tp_init` (:meth:`~object.__init__`).
+
+   This cannot be used for objects with :c:macro:`Py_TPFLAGS_HAVE_GC` set in
+   :c:member:`~PyTypeObject.tp_flags`; use :c:macro:`PyObject_GC_New` instead.
+
+   Memory allocated by this function must be freed with :c:func:`PyObject_Free`
+   (usually called via the object's :c:member:`~PyTypeObject.tp_free` slot).
+
+   .. warning::
+
+      The returned memory is not guaranteed to have been completely zeroed
+      before it was initialized.
+
+   .. warning::
+
+      This macro does not construct a fully initialized object of the given
+      type; it merely allocates memory and prepares it for further
+      initialization by :c:member:`~PyTypeObject.tp_init`.  To construct a
+      fully initialized object, call *typeobj* instead.  For example::
+
+         PyObject *foo = PyObject_CallNoArgs((PyObject *)&PyFoo_Type);
 
-   Note that this function is unsuitable if *typeobj* has
-   :c:macro:`Py_TPFLAGS_HAVE_GC` set. For such objects,
-   use :c:func:`PyObject_GC_New` instead.
+   .. seealso::
+
+      * :c:func:`PyObject_Free`
+      * :c:macro:`PyObject_GC_New`
+      * :c:func:`PyType_GenericAlloc`
+      * :c:member:`~PyTypeObject.tp_alloc`
 
 
 .. c:macro:: PyObject_NewVar(TYPE, typeobj, size)
 
-   Allocate a new Python object using the C structure type *TYPE* and the
-   Python type object *typeobj* (``PyTypeObject*``).
-   Fields not defined by the Python object header
-   are not initialized.  The allocated memory allows for the *TYPE* structure
-   plus *size* (``Py_ssize_t``) fields of the size
-   given by the :c:member:`~PyTypeObject.tp_itemsize` field of
-   *typeobj*.  This is useful for implementing objects like tuples, which are
-   able to determine their size at construction time.  Embedding the array of
-   fields into the same allocation decreases the number of allocations,
-   improving the memory management efficiency.
-
-   Note that this function is unsuitable if *typeobj* has
-   :c:macro:`Py_TPFLAGS_HAVE_GC` set. For such objects,
-   use :c:func:`PyObject_GC_NewVar` instead.
+   Like :c:macro:`PyObject_New` except:
+
+   * It allocates enough memory for the *TYPE* structure plus *size*
+     (``Py_ssize_t``) fields of the size given by the
+     :c:member:`~PyTypeObject.tp_itemsize` field of *typeobj*.
+   * The memory is initialized like :c:func:`PyObject_InitVar`.
+
+   This is useful for implementing objects like tuples, which are able to
+   determine their size at construction time.  Embedding the array of fields
+   into the same allocation decreases the number of allocations, improving the
+   memory management efficiency.
+
+   Avoid calling this directly to allocate memory for an object; call the type's
+   :c:member:`~PyTypeObject.tp_alloc` slot instead.
+
+   When populating a type's :c:member:`~PyTypeObject.tp_alloc` slot,
+   :c:func:`PyType_GenericAlloc` is preferred over a custom function that
+   simply calls this macro.
+
+   This cannot be used for objects with :c:macro:`Py_TPFLAGS_HAVE_GC` set in
+   :c:member:`~PyTypeObject.tp_flags`; use :c:macro:`PyObject_GC_NewVar`
+   instead.
+
+   Memory allocated by this function must be freed with :c:func:`PyObject_Free`
+   (usually called via the object's :c:member:`~PyTypeObject.tp_free` slot).
+
+   .. warning::
+
+      The returned memory is not guaranteed to have been completely zeroed
+      before it was initialized.
+
+   .. warning::
+
+      This macro does not construct a fully initialized object of the given
+      type; it merely allocates memory and prepares it for further
+      initialization by :c:member:`~PyTypeObject.tp_init`.  To construct a
+      fully initialized object, call *typeobj* instead.  For example::
+
+         PyObject *list_instance = PyObject_CallNoArgs((PyObject *)&PyList_Type);
+
+   .. seealso::
+
+      * :c:func:`PyObject_Free`
+      * :c:macro:`PyObject_GC_NewVar`
+      * :c:func:`PyType_GenericAlloc`
+      * :c:member:`~PyTypeObject.tp_alloc`
 
 
 .. c:function:: void PyObject_Del(void *op)

Doc/c-api/gcsupport.rst

@@ -57,11 +57,49 @@ rules:
    Analogous to :c:macro:`PyObject_New` but for container objects with the
    :c:macro:`Py_TPFLAGS_HAVE_GC` flag set.
 
+   Do not call this directly to allocate memory for an object; call the type's
+   :c:member:`~PyTypeObject.tp_alloc` slot instead.
+
+   When populating a type's :c:member:`~PyTypeObject.tp_alloc` slot,
+   :c:func:`PyType_GenericAlloc` is preferred over a custom function that
+   simply calls this macro.
+
+   Memory allocated by this function must be freed with
+   :c:func:`PyObject_GC_Del` (usually called via the object's
+   :c:member:`~PyTypeObject.tp_free` slot).
+
+   .. seealso::
+
+      * :c:func:`PyObject_GC_Del`
+      * :c:macro:`PyObject_New`
+      * :c:func:`PyType_GenericAlloc`
+      * :c:member:`~PyTypeObject.tp_alloc`
+
+
 .. c:macro:: PyObject_GC_NewVar(TYPE, typeobj, size)
 
    Analogous to :c:macro:`PyObject_NewVar` but for container objects with the
    :c:macro:`Py_TPFLAGS_HAVE_GC` flag set.
 
+   Do not call this directly to allocate memory for an object; call the type's
+   :c:member:`~PyTypeObject.tp_alloc` slot instead.
+
+   When populating a type's :c:member:`~PyTypeObject.tp_alloc` slot,
+   :c:func:`PyType_GenericAlloc` is preferred over a custom function that
+   simply calls this macro.
+
+   Memory allocated by this function must be freed with
+   :c:func:`PyObject_GC_Del` (usually called via the object's
+   :c:member:`~PyTypeObject.tp_free` slot).
+
+   .. seealso::
+
+      * :c:func:`PyObject_GC_Del`
+      * :c:macro:`PyObject_NewVar`
+      * :c:func:`PyType_GenericAlloc`
+      * :c:member:`~PyTypeObject.tp_alloc`
+
+
 .. c:function:: PyObject* PyUnstable_Object_GC_NewWithExtraData(PyTypeObject *type, size_t extra_size)
 
    Analogous to :c:macro:`PyObject_GC_New` but allocates *extra_size*
@@ -73,6 +111,10 @@ rules:
    The extra data will be deallocated with the object, but otherwise it is
    not managed by Python.
 
+   Memory allocated by this function must be freed with
+   :c:func:`PyObject_GC_Del` (usually called via the object's
+   :c:member:`~PyTypeObject.tp_free` slot).
+
    .. warning::
       The function is marked as unstable because the final mechanism
       for reserving extra data after an instance is not yet decided.
@@ -136,6 +178,21 @@ rules:
    Releases memory allocated to an object using :c:macro:`PyObject_GC_New` or
    :c:macro:`PyObject_GC_NewVar`.
 
+   Do not call this directly to free an object's memory; call the type's
+   :c:member:`~PyTypeObject.tp_free` slot instead.
+
+   Do not use this for memory allocated by :c:macro:`PyObject_New`,
+   :c:macro:`PyObject_NewVar`, or related allocation functions; use
+   :c:func:`PyObject_Free` instead.
+
+   .. seealso::
+
+      * :c:func:`PyObject_Free` is the non-GC equivalent of this function.
+      * :c:macro:`PyObject_GC_New`
+      * :c:macro:`PyObject_GC_NewVar`
+      * :c:func:`PyType_GenericAlloc`
+      * :c:member:`~PyTypeObject.tp_free`
+
 
 .. c:function:: void PyObject_GC_UnTrack(void *op)