PEP 728: Expand previous discussions and "How to teach this" (#4539)

The SC reached out to ask for more motivation and a better "How to teach this" section.
I gathered some previous discussions of this feature and rewrote the teaching section.

Author: JelleZijlstra

peps/pep-0728.rst

@@ -123,6 +123,24 @@ Given the usage of pre-:pep:`692` type annotation for ``**kwargs`` in existing
 codebases, it will be valuable to accept and type extra items on TypedDict so
 that the old typing behavior can be supported in combination with ``Unpack``.
 
+Previous Discussions
+--------------------
+
+The new features introduced in this PEP would address several long-standing feature
+requests in the type system. Previous discussions include:
+
+- `Mypy issue <https://github.com/python/mypy/issues/7981>`__ asking for a "final TypedDict"
+  (2019). While the discussion focuses on the ``@final`` decorator, the underlying feature request
+  would be addressed by this PEP.
+- `Mailing list thread <https://mail.python.org/archives/list/typing-sig@python.org/thread/66RITIHDQHVTUMJHH2ORSNWZ6DOPM367/>`__
+  asking for a way to say that a ``TypedDict`` can contain arbitrary extra keys (2020).
+- `Discussion <https://discuss.python.org/t/pep-692-using-typeddict-for-more-precise-kwargs-typing/17314/87>`__
+  about an extension of the ``Unpack`` mechanism introduced by :pep:`692` (2023).
+- `PEP 705 <https://discuss.python.org/t/pep-705-typeddict-read-only-and-other-keys/36457>`__ in an earlier
+  draft proposed a similar feature (2023); it was removed to keep that PEP simpler.
+- `Discussion <https://discuss.python.org/t/do-we-want-an-exact-typeddict-if-so-how-final-extras-never/44418>`__
+  about an "exact" ``TypedDict`` (2024).
+
 Rationale
 =========
 
@@ -747,12 +765,22 @@ valid definition that indicates all extra items must be ``None``.)
 How to Teach This
 =================
 
-The choice of the spelling ``"extra_items"`` is intended to make this
-feature more understandable to new users compared to shorter alternatives like
-``"extra"``.
-
-Details of this should be documented in both the typing spec and the
-:mod:`typing` documentation.
+The new features introduced in this PEP can be taught together with the concept
+of inheritance as it applies to ``TypedDict``. A possible outline could be:
+
+- Basics of ``TypedDict``: a ``dict`` with a fixed set of keys and value types.
+- ``NotRequired``, ``Required``, and ``total=False``: keys that may be missing.
+- ``ReadOnly``: keys that cannot be modified.
+- Inheritance: subclasses can add new keys. As a corollary, a value of a ``TypedDict``
+  type may contain additional keys at runtime that are not specified in the type.
+- ``closed=True``: disallowing additional keys and restricting inheritance.
+- ``extra_items=VT``: allowing additional keys with a specified value type.
+
+The concept of a closed ``TypedDict`` should also be cross-referenced in documentation
+for related concepts. For example, type narrowing with the ``in`` operator works
+differently, perhaps more intuitively, with closed ``TypedDict`` types. In addition, when ``Unpack``
+is used for keyword arguments, a closed ``TypedDict`` can be useful to restrict the
+allowed keyword arguments.
 
 Backwards Compatibility
 =======================