gh-141984: Move generator iterator reference out of syntax docs

Doc/library/stdtypes.rst

@@ -891,10 +891,12 @@ many numeric contexts, ``False`` and ``True`` behave like the integers 0 and 1,
 However, relying on this is discouraged; explicitly convert using :func:`int`
 instead.
 
+.. _iterator-types:
+
 .. _typeiter:
 
-Iterator Types
-==============
+Iteration-related types
+=======================
 
 .. index::
    single: iterator protocol
@@ -907,6 +909,9 @@ 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.
 
+Iterables
+---------
+
 One method needs to be defined for container objects to provide :term:`iterable`
 support:
 
@@ -923,10 +928,14 @@ support:
    :c:member:`~PyTypeObject.tp_iter` slot of the type structure for Python
    objects in the Python/C API.
 
+.. _stdtypes-iterators:
+
+Iterators
+---------
+
 The iterator objects themselves are required to support the following two
 methods, which together form the :dfn:`iterator protocol`:
 
-
 .. method:: iterator.__iter__()
 
    Return the :term:`iterator` object itself.  This is required to allow both
@@ -955,16 +964,278 @@ Implementations that do not obey this property are deemed broken.
 
 .. _generator-types:
 
-Generator Types
+Generator types
 ---------------
 
-Python's :term:`generator`\s provide a convenient way to implement the iterator
-protocol.  If a container object's :meth:`~object.__iter__` method is implemented as a
-generator, it will automatically return an iterator object (technically, a
-generator object) supplying the :meth:`~iterator.__iter__` and :meth:`~generator.__next__`
-methods.
-More information about generators can be found in :ref:`the documentation for
-the yield expression <yieldexpr>`.
+Python's :term:`generators <generator>` -- or more precisely,
+:term:`generator functions <generator function>` and
+:term:`generator iterators <generator iterator>` -- provide a convenient way
+to implement the iterator protocol.
+
+A function that contains one or more :ref:`yield expressions <yieldexpr>`
+is a :term:`generator function`.
+For example::
+
+   >>> def count_to_three():
+   ...     yield 0
+   ...     yield 1
+   ...     yield 2
+   ...     yield 3
+
+Generator functions behave as regular
+:ref:`user-defined functions <user-defined-funcs>`
+(for example, they have the same attributes), except that calling a generator
+function returns a :ref:`generator iterator <generator-methods>`::
+
+   >>> count_to_three()
+   <generator object count_to_three at 0x7f33a2305000>
+
+Iterating a generator iterator executes code of the underlying
+generator function, producing each :keyword:`yield`\ed value in turn::
+
+   >>> for number in count_to_three():
+   ...     print(number)
+   0
+   1
+   2
+   3
+
+   >>> list(count_to_three())
+   [0, 1, 2, 3]
+
+One common use for generator functions is implementing the
+:meth:`~object.__iter__` method of custom iterable objects.
+For example::
+
+   >>> class CardDeck:
+   ...     def __iter__(self):
+   ...         yield 'three of clubs'
+   ...         yield 'ace of hearts'
+
+   >>> list(CardDeck())
+   ['three of clubs', 'ace of hearts']
+
+
+.. index:: pair: object; generator
+.. _generator-methods:
+
+Generator iterators
+^^^^^^^^^^^^^^^^^^^
+
+Generator iterators implement the
+:ref:`iterator protocol <stdtypes-iterators>`.
+Iterating them drives execution of the underlying generator function.
+
+.. index:: pair: exception; StopIteration
+
+.. method:: generator.__next__()
+
+   Starts the execution of a generator function or resumes it at the
+   :ref:`yield expression <yieldexpr>` where the function is currently suspended.
+   When a generator function is resumed with a :meth:`~generator.__next__`
+   method, the current yield expression always evaluates to :const:`None`.
+   The execution then continues to the next yield expression, where the
+   generator is suspended again, and the value of the expression after the
+   :keyword:`yield` keyword is returned to :meth:`~generator.__next__`'s
+   caller.
+   If the generator exits without yielding another value,
+   :meth:`~generator.__next__` raises a :exc:`StopIteration` exception,
+   signalling that iteration has completed.
+
+   This method is normally called implicitly, for example by a :keyword:`for`
+   loop, or by the built-in :func:`next` function.
+
+Generator iterators have a few more methods than generic iterators, which
+can be used to control the execution of the underlying generator function:
+
+.. method:: generator.send(value)
+
+   "Sends" a value into the generator function: the *value* argument becomes
+   the result of the current yield expression.
+
+   Otherwise, this method behaves like :meth:`~generator.__next__`: it resumes
+   the underlying function and either returns the next yielded value or raises
+   :exc:`StopIteration`.
+
+   When :meth:`send` is called to start the generator, it must be called
+   with :const:`None` as the argument, because there is no current yield
+   expression that could receive the value.
+
+
+.. method:: generator.throw(value)
+            generator.throw(type[, value[, traceback]])
+
+   Raises an exception at the point where the generator is currently suspended.
+
+   Otherwise, this method behaves like :meth:`~generator.__next__`: it resumes
+   the underlying function and either returns the next yielded value or raises
+   :exc:`StopIteration`.
+   If the generator function does not catch the passed-in exception, or
+   raises a different exception, then that exception propagates to the caller.
+
+   When :meth:`throw` is called to start the generator, the generator
+   immediately exits (that is, subsequent calls to :meth:`~generator.__next__`
+   will raise :exc:`StopIteration`) and the thrown exception is propagated to
+   :meth:`throw`'s caller.
+
+   In typical use, this is called with a single argument, an exception instance,
+   similar to the way the :keyword:`raise` keyword is used.
+
+   For backwards compatibility, however, the second signature is
+   supported, following a convention from older versions of Python.
+   The *type* argument should be an exception class, and *value*
+   should be an exception instance. If the *value* is not provided, the
+   *type* constructor is called to get an instance. If *traceback*
+   is provided, it is set on the exception, otherwise any existing
+   :attr:`~BaseException.__traceback__` attribute stored in *value* may
+   be cleared.
+
+   .. versionchanged:: 3.12
+
+      The second signature \(type\[, value\[, traceback\]\]\) is deprecated and
+      may be removed in a future version of Python.
+
+.. index:: pair: exception; GeneratorExit
+
+.. method:: generator.close()
+
+   Raises a :exc:`GeneratorExit` exception at the point where the generator
+   function is currently suspended (equivalent to calling ``throw(GeneratorExit)``).
+
+   If the generator function has already exited (due to an exception or
+   normal return), or raises :exc:`GeneratorExit` (by not catching the
+   exception), :meth:`close` returns :const:`None`.
+   If the generator yields a value, a :exc:`RuntimeError` is raised.
+   If the generator raises any other exception, it is propagated to the caller.
+   If a generator returns a value upon being closed, that value is returned
+   by :meth:`close`.
+
+   When a generator iterator is garbage collected before it has exited,
+   :meth:`~generator.close` is called automatically.
+
+   .. versionchanged:: 3.13
+
+      If a generator returns a value upon being closed, the value is returned
+      by :meth:`close`.
+      Previously, it returned ``None``.
+
+
+Calling any of the generator methods (:meth:`~generator.__next__`,
+:meth:`~generator.send`, :meth:`~generator.throw`, :meth:`~generator.close`)
+while one of these methods is already executing
+raises a :exc:`ValueError` exception.
+
+
+.. index:: pair: object; asynchronous-generator
+.. _asynchronous-generator-methods:
+
+Asynchronous generator iterators
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This subsection describes the methods of an asynchronous generator iterator,
+which are used to control the execution of an asynchronous generator function.
+
+
+.. index:: pair: exception; StopAsyncIteration
+
+.. method:: agen.__anext__()
+   :async:
+
+   Returns an :term:`awaitable` which when run starts to execute the
+   asynchronous generator function or resumes it at the
+   :ref:`yield expression <yieldexpr>` where the function is currently suspended.
+   When an asynchronous generator function is resumed with an
+   :meth:`~agen.__anext__` method, the current yield expression always
+   evaluates to :const:`None` in the returned awaitable, which when run will
+   continue to the next yield expression.
+   The value of the expression after the :keyword:`yield` keyword is the value
+   of the :exc:`StopIteration` exception raised by the completing coroutine.
+   If the asynchronous generator exits without yielding another value, the
+   awaitable instead raises a :exc:`StopAsyncIteration` exception,
+   signalling that the asynchronous iteration has completed.
+
+   This method is normally called implicitly by an :keyword:`async for` loop,
+   or by the built-in :func:`anext` function.
+
+
+Asynchronous generator-iterators have a few more methods than generic
+asynchronous iterators, which can be used to control the execution of
+the underlying generator function:
+
+.. method:: agen.asend(value)
+   :async:
+
+   Returns an awaitable which, when run, "sends" a value into the underlying
+   asynchronous generator function: the *value* argument becomes
+   the result of the current yield expression.
+
+   Otherwise, this method behaves like :meth:`~agen.__anext__`: when the
+   returned awaitable runs, it resumes the underlying function and either
+   returns the next yielded value as the value of the raised
+   :exc:`StopIteration`, or raises :exc:`StopAsyncIteration`.
+
+   When :meth:`asend` is called to start the asynchronous
+   generator, it must be called with :const:`None` as the argument,
+   because there is no yield expression that could receive the value.
+
+
+.. method:: agen.athrow(value)
+            agen.athrow(type[, value[, traceback]])
+   :async:
+
+   Returns an awaitable that, when run, raises an exception at the point where
+   the underlying asynchronous generator function is currently suspended.
+
+   Otherwise, this method behaves like :meth:`~agen.__anext__`: when the
+   returned awaitable runs, it resumes the underlying function (with an
+   exception raised) and either returns the next yielded value as the value of
+   the raised :exc:`StopIteration`, or raises :exc:`StopAsyncIteration`.
+   If the underlying function does not catch the passed-in exception, or
+   raises a different exception, then when the awaitable is run, that
+   exception propagates to the caller of the awaitable.
+
+   When :meth:`~agen.athrow` is called to start the generator, the generator
+   exits when the awaitable runs (that is, subsequent results from
+   :meth:`~agen.__anext__` will raise :exc:`StopAsyncIteration` when run)
+   and the thrown exception is propagated to the awaitable's caller.
+
+   In typical use, this is called with a single argument, an exception instance,
+   similar to the way the :keyword:`raise` keyword is used.
+
+   For backwards compatibility, however, the second signature is
+   supported.
+   An exception instance is created from three arguments in the same way as in
+   :meth:`generator.throw`.
+
+   .. versionchanged:: 3.12
+
+      The second signature \(type\[, value\[, traceback\]\]\) is deprecated and
+      may be removed in a future version of Python.
+
+
+.. index:: pair: exception; GeneratorExit
+
+.. method:: agen.aclose()
+   :async:
+
+   Returns an awaitable that when run will throw a :exc:`GeneratorExit` into
+   the underlying asynchronous generator function at the point where it is
+   currently suspended (equivalent to calling ``athrow(GeneratorExit)``).
+
+   If the asynchronous generator function then exits gracefully, is already
+   closed, or raises :exc:`GeneratorExit` (by not catching the exception),
+   then the returned awaitable will raise a :exc:`StopIteration` exception.
+   Any further awaitables returned by subsequent calls to the asynchronous
+   generator will raise a :exc:`StopAsyncIteration` exception.
+
+   If the asynchronous generator yields a value, a :exc:`RuntimeError` is
+   raised by the awaitable.
+   If the asynchronous generator raises any other exception, that exception
+   is propagated to the caller of the awaitable.
+
+   If the asynchronous generator has already exited due to an exception or
+   normal exit, then further calls to :meth:`aclose` will return an awaitable
+   that does nothing.
 
 
 .. _typesseq: