bpo-45250: fix docs regarding __iter__ and iterators being inconsistently required by CPython (GH-29170)

It is now considered a historical accident that e.g. `for` loops and the `iter()` built-in function do not require the iterators they work with to define `__iter__`, only `__next__`.

Author: brettcannon

Doc/c-api/iter.rst

@@ -9,8 +9,8 @@ There are two functions specifically for working with iterators.
 
 .. c:function:: int PyIter_Check(PyObject *o)
 
-   Return non-zero if the object *o* supports the iterator protocol, and ``0``
-   otherwise.  This function always succeeds.
+   Return non-zero if the object *o* can be safely passed to
+   :c:func:`PyIter_Next`, and ``0`` otherwise.  This function always succeeds.
 
 .. c:function:: int PyAIter_Check(PyObject *o)
 
@@ -21,10 +21,11 @@ There are two functions specifically for working with iterators.
 
 .. c:function:: PyObject* PyIter_Next(PyObject *o)
 
-   Return the next value from the iteration *o*.  The object must be an iterator
-   (it is up to the caller to check this).  If there are no remaining values,
-   returns ``NULL`` with no exception set.  If an error occurs while retrieving
-   the item, returns ``NULL`` and passes along the exception.
+   Return the next value from the iterator *o*.  The object must be an iterator
+   according to :c:func:`PyIter_Check` (it is up to the caller to check this).
+   If there are no remaining values, returns ``NULL`` with no exception set.
+   If an error occurs while retrieving the item, returns ``NULL`` and passes
+   along the exception.
 
 To write a loop which iterates over an iterator, the C code should look
 something like this::

Doc/c-api/typeobj.rst

@@ -1521,9 +1521,9 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
 .. c:member:: getiterfunc PyTypeObject.tp_iter
 
-   An optional pointer to a function that returns an iterator for the object.  Its
-   presence normally signals that the instances of this type are iterable (although
-   sequences may be iterable without this function).
+   An optional pointer to a function that returns an :term:`iterator` for the
+   object.  Its presence normally signals that the instances of this type are
+   :term:`iterable` (although sequences may be iterable without this function).
 
    This function has the same signature as :c:func:`PyObject_GetIter`::
 
@@ -1536,8 +1536,8 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
 .. c:member:: iternextfunc PyTypeObject.tp_iternext
 
-   An optional pointer to a function that returns the next item in an iterator.
-   The signature is::
+   An optional pointer to a function that returns the next item in an
+   :term:`iterator`. The signature is::
 
       PyObject *tp_iternext(PyObject *self);
 
@@ -2429,8 +2429,8 @@ Async Object Structures
 
       PyObject *am_await(PyObject *self);
 
-   The returned object must be an iterator, i.e. :c:func:`PyIter_Check` must
-   return ``1`` for it.
+   The returned object must be an :term:`iterator`, i.e. :c:func:`PyIter_Check`
+   must return ``1`` for it.
 
    This slot may be set to ``NULL`` if an object is not an :term:`awaitable`.
 

Doc/glossary.rst

@@ -659,6 +659,11 @@ Glossary
 
       More information can be found in :ref:`typeiter`.
 
+      .. impl-detail::
+
+         CPython does not consistently apply the requirement that an iterator
+         define :meth:`__iter__`.
+
    key function
       A key function or collation function is a callable that returns a value
       used for sorting or ordering.  For example, :func:`locale.strxfrm` is

Doc/library/functions.rst

@@ -66,9 +66,6 @@ are always available.  They are listed here in alphabetical order.
    Return an :term:`asynchronous iterator` for an :term:`asynchronous iterable`.
    Equivalent to calling ``x.__aiter__()``.
 
-   ``aiter(x)`` itself has an ``__aiter__()`` method that returns ``x``,
-   so ``aiter(aiter(x))`` is the same as ``aiter(x)``.
-
    Note: Unlike :func:`iter`, :func:`aiter` has no 2-argument variant.
 
    .. versionadded:: 3.10
@@ -929,8 +926,8 @@ are always available.  They are listed here in alphabetical order.
    Return an :term:`iterator` object.  The first argument is interpreted very
    differently depending on the presence of the second argument. Without a
    second argument, *object* must be a collection object which supports the
-   iteration protocol (the :meth:`__iter__` method), or it must support the
-   sequence protocol (the :meth:`__getitem__` method with integer arguments
+   :term:`iterable` protocol (the :meth:`__iter__` method), or it must support
+   the sequence protocol (the :meth:`__getitem__` method with integer arguments
    starting at ``0``).  If it does not support either of those protocols,
    :exc:`TypeError` is raised. If the second argument, *sentinel*, is given,
    then *object* must be a callable object.  The iterator created in this case
@@ -1060,7 +1057,7 @@ are always available.  They are listed here in alphabetical order.
 
 .. function:: next(iterator[, default])
 
-   Retrieve the next item from the *iterator* by calling its
+   Retrieve the next item from the :term:`iterator` by calling its
    :meth:`~iterator.__next__` method.  If *default* is given, it is returned
    if the iterator is exhausted, otherwise :exc:`StopIteration` is raised.
 

Doc/library/stdtypes.rst

@@ -810,40 +810,41 @@ using two distinct methods; these are used to allow user-defined classes to
 support iteration.  Sequences, described below in more detail, always support
 the iteration methods.
 
-One method needs to be defined for container objects to provide iteration
+One method needs to be defined for container objects to provide :term:`iterable`
 support:
 
 .. XXX duplicated in reference/datamodel!
 
 .. method:: container.__iter__()
 
-   Return an iterator object.  The object is required to support the iterator
-   protocol described below.  If a container supports different types of
-   iteration, additional methods can be provided to specifically request
+   Return an :term:`iterator` object.  The object is required to support the
+   iterator protocol described below.  If a container supports different types
+   of iteration, additional methods can be provided to specifically request
    iterators for those iteration types.  (An example of an object supporting
    multiple forms of iteration would be a tree structure which supports both
    breadth-first and depth-first traversal.)  This method corresponds to the
-   :c:member:`~PyTypeObject.tp_iter` slot of the type structure for Python objects in the Python/C
-   API.
+   :c:member:`~PyTypeObject.tp_iter` slot of the type structure for Python
+   objects in the Python/C API.
 
 The iterator objects themselves are required to support the following two
 methods, which together form the :dfn:`iterator protocol`:
 
 
 .. method:: iterator.__iter__()
 
-   Return the iterator object itself.  This is required to allow both containers
-   and iterators to be used with the :keyword:`for` and :keyword:`in` statements.
-   This method corresponds to the :c:member:`~PyTypeObject.tp_iter` slot of the type structure for
-   Python objects in the Python/C API.
+   Return the :term:`iterator` object itself.  This is required to allow both
+   containers and iterators to be used with the :keyword:`for` and
+   :keyword:`in` statements.  This method corresponds to the
+   :c:member:`~PyTypeObject.tp_iter` slot of the type structure for Python
+   objects in the Python/C API.
 
 
 .. method:: iterator.__next__()
 
-   Return the next item from the container.  If there are no further items, raise
-   the :exc:`StopIteration` exception.  This method corresponds to the
-   :c:member:`~PyTypeObject.tp_iternext` slot of the type structure for Python objects in the
-   Python/C API.
+   Return the next item from the :term:`iterator`.  If there are no further
+   items, raise the :exc:`StopIteration` exception.  This method corresponds to
+   the :c:member:`~PyTypeObject.tp_iternext` slot of the type structure for
+   Python objects in the Python/C API.
 
 Python defines several iterator objects to support iteration over general and
 specific sequence types, dictionaries, and other more specialized forms.  The

Doc/reference/datamodel.rst

@@ -648,13 +648,13 @@ Callable types
 
       A function or method which uses the :keyword:`yield` statement (see section
       :ref:`yield`) is called a :dfn:`generator function`.  Such a function, when
-      called, always returns an iterator object which can be used to execute the
-      body of the function:  calling the iterator's :meth:`iterator.__next__`
-      method will cause the function to execute until it provides a value
-      using the :keyword:`!yield` statement.  When the function executes a
-      :keyword:`return` statement or falls off the end, a :exc:`StopIteration`
-      exception is raised and the iterator will have reached the end of the set of
-      values to be returned.
+      called, always returns an :term:`iterator` object which can be used to
+      execute the body of the function:  calling the iterator's
+      :meth:`iterator.__next__` method will cause the function to execute until
+      it provides a value using the :keyword:`!yield` statement.  When the
+      function executes a :keyword:`return` statement or falls off the end, a
+      :exc:`StopIteration` exception is raised and the iterator will have
+      reached the end of the set of values to be returned.
 
    Coroutine functions
       .. index::
@@ -674,7 +674,7 @@ Callable types
       A function or method which is defined using :keyword:`async def` and
       which uses the :keyword:`yield` statement is called a
       :dfn:`asynchronous generator function`.  Such a function, when called,
-      returns an asynchronous iterator object which can be used in an
+      returns an :term:`asynchronous iterator` object which can be used in an
       :keyword:`async for` statement to execute the body of the function.
 
       Calling the asynchronous iterator's :meth:`aiterator.__anext__` method
@@ -2499,12 +2499,10 @@ through the object's keys; for sequences, it should iterate through the values.
 
 .. method:: object.__iter__(self)
 
-   This method is called when an iterator is required for a container. This method
-   should return a new iterator object that can iterate over all the objects in the
-   container.  For mappings, it should iterate over the keys of the container.
-
-   Iterator objects also need to implement this method; they are required to return
-   themselves.  For more information on iterator objects, see :ref:`typeiter`.
+   This method is called when an :term:`iterator` is required for a container.
+   This method should return a new iterator object that can iterate over all the
+   objects in the container.  For mappings, it should iterate over the keys of
+   the container.
 
 
 .. method:: object.__reversed__(self)

Misc/NEWS.d/next/Documentation/2021-10-22-12-09-18.bpo-45250.Iit5-Y.rst

@@ -0,0 +1,2 @@
+Update the documentation to note that CPython does not consistently
+require iterators to define ``__iter__``.