PEP 802: Add Reference Implementation, expand Rejected Ideas (#4537)

Author: AA-Turner

peps/pep-0802.rst

@@ -41,8 +41,10 @@ of core concepts to beginners.
 For example, users must be careful not to use ``set`` as a local variable name,
 as doing so prevents constructing new sets.
 This can be frustrating as beginners may not know how to recover the
-:py:class:`set` type if they have overriden the name:
-techniques to do so (e.g. ``type({1}-{1})``) are not immediately obvious.
+:py:class:`set` type if they have overriden the name.
+Techniques to do so (e.g. ``type({1})``) are not immediately obvious,
+especially to those learning the language, who may not yet be familiar
+with the :py:class:`python:type` function.
 
 Finally, this may be helpful for users who do not speak English,
 as it provides a culture-free notation for a common data structure
@@ -79,6 +81,8 @@ with various proposals, including:
    (`Discourse, August 2025 (#37)`_)
 #. ``{-}``
    (`python-ideas, August 2020`_)
+#. ``(/)``
+   (`Discourse, March 2023 (#20)`_)
 #. ``{,}``
    (`Discourse, August 2025`_)
 #. ``{/}``
@@ -106,6 +110,10 @@ The grammar for set displays will become:
 .. productionlist:: python-grammar
    set_display: "{" ("/" | flexible_expression_list | comprehension) "}"
 
+The parser will emit a specialised error message for ``(/)`` and ``[/]``,
+indicating that these are invalid syntax, and suggesting the correct forms
+of the empty tuple and list respectively.
+
 ``{/}`` will become the default syntax for the empty set.
 
 .. code-block:: python
@@ -141,6 +149,12 @@ There will be no other backwards incompatibile changes,
 all current constructors for the empty set will continue to work,
 and the behaviour of the :py:class:`set` type will remain unchanged.
 
+Due to this, current code that uses ``set()`` will continue to work,
+and can remain unchanged.
+Developers of automated tools such as linters and formatters should avoid
+encouraging 'churn' in existing projects by suggesting changes en-masse from
+``set()`` to ``{/}``, unless explicitly requested by the user of such tools.
+
 
 Security Implications
 =====================
@@ -172,7 +186,9 @@ especially those that have a maths or science background.
 Reference Implementation
 ========================
 
-Forthcoming.
+A reference implementation of this PEP exists as a pull request
+in the CPython repository on Github:
+`python/cpython#137565 <https://github.com/python/cpython/pull/137565>`__.
 
 
 Rejected Ideas
@@ -217,8 +233,23 @@ Use the ``{*()}`` syntax
 ------------------------
 
 This relies on unpacking the empty tuple into a set, creating an empty set.
-This has the benefit of support since Python 3.5 (:pep:`448`), but is unwieldy
-and complicated to explain; it was not designed for this purpose.
+This has the benefit of support since Python 3.5 (:pep:`448`).
+
+Explaining this notation requires understanding sequence unpacking, the empty
+tuple, set literal syntax, and the concept of unpacking an empty sequence,
+producing no values to unpack.
+
+This is an unwieldy syntax; not one that can be easily taught to beginners.
+We should avoid introducing complex notation with the proviso that users
+will understand it later, or that it can be looked up online.
+This removes agency from the learner.
+Especially when using a programming language that can execute arbitrary code,
+it is important that users understand what the code they are writing does.
+
+In the authors' opinion, it would be a misttep to promote ``{*()}``
+as the syntax for an empty set.
+It resulted as a side-effect of :pep:`448`, rather than an intentional attempt
+to design a syntax that is easy to teach, understand, and explain.
 
 Use the ``{-}`` syntax
 ----------------------
@@ -227,6 +258,20 @@ This syntax was originally proposed in :pep:`218`,
 but removed from the PEP before it was accepted.
 The authors prefer ``{/}`` due to the resemblance to :math:`\emptyset`.
 
+Use the ``(/)`` syntax
+----------------------
+
+This notation has been suggested as a better visual resemlence of
+the empty set symbol (e.g. U+2205 ∅ EMPTY SET).
+However, it is a complete departure from the regular set syntax.
+To the authors, any of the proposed notations with curly brackets
+are better than this proposal, as they are more consistent with the
+current (since Python 3.0) set notation.
+
+The authors prefer ``{/}`` because it combines the benefits of
+visual resemblance to the mathematical symbol and to the existing set syntax,
+unlike ``(/)``.
+
 Use the ``{,}`` syntax
 ----------------------
 
@@ -240,6 +285,19 @@ Having a visibly different form, in ``{/}``, helps to reinforce the idea
 that the syntax for the empty set is a special case, rather than a general rule
 for all empty collections.
 
+Create and use a new token for ``'{/}'``
+----------------------------------------
+
+There have been `previous proposals <https://discuss.python.org/t/73235/66>`__
+to create a new token for this construct.
+This would require ``'{/}'`` to be written literally,
+with no whitespace between the characters.
+
+We insted chose to allow whitespace between the brackets and the slash,
+treating ``{``, ``/``, and ``}`` as three distinct tokens,
+as we cannot see any reason to prevent it.
+However, we expect that the vast majority of uses will not include whitespace.
+
 
 Open Issues
 ===========
@@ -258,10 +316,11 @@ Acknowledgements
 Footnotes
 =========
 
-.. _Discourse, August 2025 (#37): https://discuss.python.org/t/101197/37
 .. _Discourse, August 2025: https://discuss.python.org/t/101197
+.. _Discourse, August 2025 (#37): https://discuss.python.org/t/101197/37
 .. _Discourse, December 2024: https://discuss.python.org/t/73235
 .. _Discourse, March 2023: https://discuss.python.org/t/25213
+.. _Discourse, March 2023 (#20): https://discuss.python.org/t/25213/20
 .. _explicitly deferred: https://mail.python.org/pipermail/python-3000/2006-May/001599.html
 .. _python-3000, April 2006: https://mail.python.org/pipermail/python-3000/2006-April/001286.html
 .. _python-3000, May 2006: https://mail.python.org/pipermail/python-3000/2006-May/001666.html