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gh-115528: Update language reference for PEP 646 (#121181)

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To recap: the objective is to make starred expressions valid in `subscription`,
which is used for generics: `Generic[...]`, `list[...]`, etc.

What _is_ gramatically valid in such contexts? Seemingly any of the following.
(At least, none of the following throw `SyntaxError` in a 3.12.3 REPL.)

    Generic[x]
    Generic[*x]
    Generic[*x, y]
    Generic[y, *x]
    Generic[x := 1]
    Generic[x := 1, y := 2]

So introducting

    flexible_expression: expression | assignment_expression | starred_item

end then switching `subscription` to use `flexible_expression` sorts that.

But then we need to field `yield` - for which any of the following are
apparently valid:

    yield x
    yield x,
    yield x, y
    yield *x,
    yield *x, *y

Introducing a separate `yield_list` is the simplest way I've been figure out to
do this - separating out the special case of `starred_item ,`.



Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
This commit is contained in:
Matthew Rahtz 2024-09-26 20:35:17 +01:00 committed by GitHub
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commit 7d3497f617
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2 changed files with 32 additions and 17 deletions
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10
Doc/reference/compound_stmts.rst
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@ -1217,9 +1217,10 @@ A function definition defines a user-defined function object (see section
: | `parameter_list_no_posonly`
parameter_list_no_posonly: `defparameter` ("," `defparameter`)* ["," [`parameter_list_starargs`]]
: | `parameter_list_starargs`
parameter_list_starargs: "*" [`parameter`] ("," `defparameter`)* ["," ["**" `parameter` [","]]]
parameter_list_starargs: "*" [`star_parameter`] ("," `defparameter`)* ["," ["**" `parameter` [","]]]
: | "**" `parameter` [","]
parameter: `identifier` [":" `expression`]
star_parameter: `identifier` [":" ["*"] `expression`]
defparameter: `parameter` ["=" `expression`]
funcname: `identifier`
@ -1326,11 +1327,16 @@ and may only be passed by positional arguments.
Parameters may have an :term:`annotation <function annotation>` of the form "``: expression``"
following the parameter name. Any parameter may have an annotation, even those of the form
``*identifier`` or ``**identifier``. Functions may have "return" annotation of
``*identifier`` or ``**identifier``. (As a special case, parameters of the form
``*identifier`` may have an annotation "``: *expression``".) Functions may have "return" annotation of
the form "``-> expression``" after the parameter list. These annotations can be
any valid Python expression. The presence of annotations does not change the
semantics of a function. See :ref:`annotations` for more information on annotations.
.. versionchanged:: 3.11
Parameters of the form "``*identifier``" may have an annotation
"``: *expression``". See :pep:`646`.
.. index:: pair: lambda; expression
It is also possible to create anonymous functions (functions not bound to a

39
Doc/reference/expressions.rst
View File

@ -284,7 +284,7 @@ A list display is a possibly empty series of expressions enclosed in square
brackets:
.. productionlist:: python-grammar
list_display: "[" [`starred_list` | `comprehension`] "]"
list_display: "[" [`flexible_expression_list` | `comprehension`] "]"
A list display yields a new list object, the contents being specified by either
a list of expressions or a comprehension. When a comma-separated list of
@ -309,7 +309,7 @@ A set display is denoted by curly braces and distinguishable from dictionary
displays by the lack of colons separating keys and values:
.. productionlist:: python-grammar
set_display: "{" (`starred_list` | `comprehension`) "}"
set_display: "{" (`flexible_expression_list` | `comprehension`) "}"
A set display yields a new mutable set object, the contents being specified by
either a sequence of expressions or a comprehension. When a comma-separated
@ -454,7 +454,7 @@ Yield expressions
.. productionlist:: python-grammar
yield_atom: "(" `yield_expression` ")"
yield_from: "yield" "from" `expression`
yield_expression: "yield" `expression_list` | `yield_from`
yield_expression: "yield" `yield_list` | `yield_from`
The yield expression is used when defining a :term:`generator` function
or an :term:`asynchronous generator` function and
@ -485,9 +485,9 @@ When a generator function is called, it returns an iterator known as a
generator. That generator then controls the execution of the generator
function. The execution starts when one of the generator's methods is called.
At that time, the execution proceeds to the first yield expression, where it is
suspended again, returning the value of :token:`~python-grammar:expression_list`
suspended again, returning the value of :token:`~python-grammar:yield_list`
to the generator's caller,
or ``None`` if :token:`~python-grammar:expression_list` is omitted.
or ``None`` if :token:`~python-grammar:yield_list` is omitted.
By suspended, we mean that all local state is
retained, including the current bindings of local variables, the instruction
pointer, the internal evaluation stack, and the state of any exception handling.
@ -576,7 +576,7 @@ is already executing raises a :exc:`ValueError` exception.
: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
:token:`~python-grammar:expression_list` is returned to :meth:`__next__`'s
:token:`~python-grammar:yield_list` is returned to :meth:`__next__`'s
caller. If the generator exits without yielding another value, a
:exc:`StopIteration` exception is raised.
@ -695,7 +695,7 @@ how a generator object would be used in a :keyword:`for` statement.
Calling one of the asynchronous generator's methods returns an :term:`awaitable`
object, and the execution starts when this object is awaited on. At that time,
the execution proceeds to the first yield expression, where it is suspended
again, returning the value of :token:`~python-grammar:expression_list` to the
again, returning the value of :token:`~python-grammar:yield_list` to the
awaiting coroutine. As with a generator, suspension means that all local state
is retained, including the current bindings of local variables, the instruction
pointer, the internal evaluation stack, and the state of any exception handling.
@ -759,7 +759,7 @@ which are used to control the execution of a generator function.
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 :token:`~python-grammar:expression_list` of the
expression. The value of the :token:`~python-grammar:yield_list` of the
yield expression 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
@ -892,7 +892,7 @@ will generally select an element from the container. The subscription of a
:ref:`GenericAlias <types-genericalias>` object.
.. productionlist:: python-grammar
subscription: `primary` "[" `expression_list` "]"
subscription: `primary` "[" `flexible_expression_list` "]"
When an object is subscripted, the interpreter will evaluate the primary and
the expression list.
@ -904,9 +904,13 @@ primary is subscripted, the evaluated result of the expression list will be
passed to one of these methods. For more details on when ``__class_getitem__``
is called instead of ``__getitem__``, see :ref:`classgetitem-versus-getitem`.
If the expression list contains at least one comma, it will evaluate to a
:class:`tuple` containing the items of the expression list. Otherwise, the
expression list will evaluate to the value of the list's sole member.
If the expression list contains at least one comma, or if any of the expressions
are starred, the expression list will evaluate to a :class:`tuple` containing
the items of the expression list. Otherwise, the expression list will evaluate
to the value of the list's sole member.
.. versionchanged:: 3.11
Expressions in an expression list may be starred. See :pep:`646`.
For built-in objects, there are two types of objects that support subscription
via :meth:`~object.__getitem__`:
@ -1905,10 +1909,12 @@ Expression lists
single: , (comma); expression list
.. productionlist:: python-grammar
starred_expression: ["*"] `or_expr`
flexible_expression: `assignment_expression` | `starred_expression`
flexible_expression_list: `flexible_expression` ("," `flexible_expression`)* [","]
starred_expression_list: `starred_expression` ("," `starred_expression`)* [","]
expression_list: `expression` ("," `expression`)* [","]
starred_list: `starred_item` ("," `starred_item`)* [","]
starred_expression: `expression` | (`starred_item` ",")* [`starred_item`]
starred_item: `assignment_expression` | "*" `or_expr`
yield_list: `expression_list` | `starred_expression` "," [`starred_expression_list`]
.. index:: pair: object; tuple
@ -1929,6 +1935,9 @@ the unpacking.
.. versionadded:: 3.5
Iterable unpacking in expression lists, originally proposed by :pep:`448`.
.. versionadded:: 3.11
Any item in an expression list may be starred. See :pep:`646`.
.. index:: pair: trailing; comma
A trailing comma is required only to create a one-item tuple,