[3.13] gh-138307: Update the Ellipsis documentation (GH-138306) (#138439)

(cherry picked from commit 88665de)

Co-authored-by: Adorilson Bezerra <[email protected]>
Co-authored-by: Stan Ulbrych <[email protected]>
Co-authored-by: Éric <[email protected]>
Co-authored-by: Ned Batchelder <[email protected]>

Author: 5 people

Doc/glossary.rst

@@ -21,7 +21,9 @@ Glossary
         right delimiters (parentheses, square brackets, curly braces or triple
         quotes), or after specifying a decorator.
 
-      * The :const:`Ellipsis` built-in constant.
+      .. index:: single: ...; ellipsis literal
+
+      * The three dots form of the :ref:`Ellipsis <bltin-ellipsis-object>` object.
 
    abstract base class
       Abstract base classes complement :term:`duck-typing` by

Doc/library/constants.rst

@@ -62,8 +62,9 @@ A small number of constants live in the built-in namespace.  They are:
 .. index:: single: ...; ellipsis literal
 .. data:: Ellipsis
 
-   The same as the ellipsis literal "``...``". Special value used mostly in conjunction
-   with extended slicing syntax for user-defined container data types.
+   The same as the ellipsis literal "``...``", an object frequently used to
+   indicate that something is omitted. Assignment to ``Ellipsis`` is possible, but
+   assignment to  ``...`` raises a :exc:`SyntaxError`.
    ``Ellipsis`` is the sole instance of the :data:`types.EllipsisType` type.
 
 

Doc/library/stdtypes.rst

@@ -5670,13 +5670,34 @@ It is written as ``None``.
 The Ellipsis Object
 -------------------
 
-This object is commonly used by slicing (see :ref:`slicings`).  It supports no
-special operations.  There is exactly one ellipsis object, named
+This object is commonly used used to indicate that something is omitted.
+It supports no special operations.  There is exactly one ellipsis object, named
 :const:`Ellipsis` (a built-in name).  ``type(Ellipsis)()`` produces the
 :const:`Ellipsis` singleton.
 
 It is written as ``Ellipsis`` or ``...``.
 
+In typical use, ``...`` as the ``Ellipsis`` object appears in a few different
+places, for instance:
+
+- In type annotations, such as :ref:`callable arguments <annotating-callables>`
+  or :ref:`tuple elements <annotating-tuples>`.
+
+- As the body of a function instead of a :ref:`pass statement <tut-pass>`.
+
+- In third-party libraries, such as `Numpy's slicing and striding
+  <https://numpy.org/doc/stable/user/basics.indexing.html#slicing-and-striding>`_.
+
+Python also uses three dots in ways that are not ``Ellipsis`` objects, for instance:
+
+- Doctest's :const:`ELLIPSIS <doctest.ELLIPSIS>`, as a pattern for missing content.
+
+- The default Python prompt of the :term:`interactive` shell when partial input is incomplete.
+
+Lastly, the Python documentation often uses three dots in conventional English
+usage to mean omitted content, even in code examples that also use them as the
+``Ellipsis``.
+
 
 .. _bltin-notimplemented-object:
 

Doc/library/typing.rst

@@ -230,9 +230,11 @@ For example:
 
    callback: Callable[[str], Awaitable[None]] = on_update
 
+.. index:: single: ...; ellipsis literal
+
 The subscription syntax must always be used with exactly two values: the
 argument list and the return type.  The argument list must be a list of types,
-a :class:`ParamSpec`, :data:`Concatenate`, or an ellipsis. The return type must
+a :class:`ParamSpec`, :data:`Concatenate`, or an ellipsis (``...``). The return type must
 be a single type.
 
 If a literal ellipsis ``...`` is given as the argument list, it indicates that
@@ -375,8 +377,11 @@ accepts *any number* of type arguments::
    # but ``z`` has been assigned to a tuple of length 3
    z: tuple[int] = (1, 2, 3)
 
+.. index:: single: ...; ellipsis literal
+
 To denote a tuple which could be of *any* length, and in which all elements are
-of the same type ``T``, use ``tuple[T, ...]``. To denote an empty tuple, use
+of the same type ``T``, use the literal ellipsis ``...``: ``tuple[T, ...]``.
+To denote an empty tuple, use
 ``tuple[()]``. Using plain ``tuple`` as an annotation is equivalent to using
 ``tuple[Any, ...]``::
 
@@ -1154,6 +1159,8 @@ These can be used as types in annotations. They all support subscription using
 
    Special form for annotating higher-order functions.
 
+   .. index:: single: ...; ellipsis literal
+
    ``Concatenate`` can be used in conjunction with :ref:`Callable <annotating-callables>` and
    :class:`ParamSpec` to annotate a higher-order callable which adds, removes,
    or transforms parameters of another

Doc/tutorial/controlflow.rst

@@ -251,6 +251,7 @@ statements: a ``try`` statement's ``else`` clause runs when no exception
 occurs, and a loop's ``else`` clause runs when no ``break`` occurs. For more on
 the ``try`` statement and exceptions, see :ref:`tut-handling`.
 
+.. index:: single: ...; ellipsis literal
 .. _tut-pass:
 
 :keyword:`!pass` Statements
@@ -277,6 +278,12 @@ at a more abstract level.  The :keyword:`!pass` is silently ignored::
    ...     pass   # Remember to implement this!
    ...
 
+For this last case, many people use the ellipsis literal :code:`...` instead of
+:code:`pass`. This use has no special meaning to Python, and is not part of
+the language definition (you could use any constant expression here), but
+:code:`...` is used conventionally as a placeholder body as well.
+See :ref:`bltin-ellipsis-object`.
+
 
 .. _tut-match: