Allow 'closed=False'; define its behavior

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

peps/pep-0728.rst

@@ -150,9 +150,9 @@ extra items are allowed, and their value types must be assignable to the
 type expression value.
 
 An application of this is to disallow extra items. We propose to add a
-``closed`` class parameter, which only accepts ``True`` as the argument.
-It should be a runtime error when ``closed=True`` and ``extra_items`` are used
-at the same time.
+``closed`` class parameter, which only accepts a literal ``True`` or ``False``
+as the argument. It should be a runtime error when ``closed`` and
+``extra_items`` are used at the same time.
 
 Different from index signatures, the types of the known items do not need to be
 assignable to the ``extra_items`` argument.
@@ -240,16 +240,27 @@ When ``closed=True`` is set, no extra items are allowed. This is a shorthand for
 ``extra_items=Never``, because there can't be a value type that is assignable to
 :class:`~typing.Never`.
 
-Different from ``total``, only a literal ``True`` is supported as the value of
-the ``closed`` argument; ``closed`` is unset by default.
+Similar to ``total``, only a literal ``True`` or ``False`` is supported as the
+value of the ``closed`` argument; ``closed`` is ``False`` by default, which
+preserves the previous TypedDict behavior.
 
 The value of ``closed`` is not inherited through subclassing, but the
-implicitly set ``extra_items=Never`` is.
+implicitly set ``extra_items=Never`` is. It should be an error to use the
+default ``closed=False`` when subclassing a closed TypedDict type::
 
-Setting both ``closed=True`` and ``extra_items`` when defining a TypedDict type
+    class BaseMovie(TypedDict, closed=True):
+        name: str
+
+    class MovieA(BaseMovie):  # Not OK. An explicit 'closed=True' is required
+        pass
+
+    class MovieB(BaseMovie, closed=True):  # OK
+        pass
+
+Setting both ``closed`` and ``extra_items`` when defining a TypedDict type
 should always be a runtime error::
 
-    class Person(TypedDict, closed=True, extra_items=bool):  # Not OK. 'closed=True' and 'extra_items' are incompatible
+    class Person(TypedDict, closed=False, extra_items=bool):  # Not OK. 'closed' and 'extra_items' are incompatible
         name: str
 
 As a consequence of ``closed=True`` being equivalent to ``extra_items=Never``.
@@ -263,17 +274,16 @@ The same rules that apply to ``extra_items=Never`` should also apply to
     class MovieClosed(Movie, closed=True):  # OK
         pass
 
-    class MovieNever(Movie, extra_items=Never):  # OK
+    class MovieNever(Movie, extra_items=Never):  # Not OK. 'closed=True' is preferred
         pass
 
 This will be further discussed in
 :ref:`a later section <pep728-inheritance-read-only>`.
 
 When neither ``extra_items`` nor ``closed=True`` is specified, the TypedDict
-type is considered non-closed. For such non-closed TypedDict types,
-it is assumed that they allow non-required extra items of value type
-``ReadOnly[object]`` during inheritance or assignability checks.
-This preserves the existing behavior of TypedDict.
+is assumed to allow non-required extra items of value type ``ReadOnly[object]``
+during inheritance or assignability checks. This preserves the existing behavior
+of TypedDict.
 
 Interaction with Totality
 -------------------------