PEP 728: Address Review Feedback (#3697)

peps/pep-0728.rst

@@ -252,7 +252,11 @@ be rejected by the type checker.
         __extra_items__: str  # A regular key
     
     a: Movie = {"name": "Blade Runner", "__extra_items__": None}  # Not OK. 'None' is incompatible with 'str'
-    b: Movie = {"name": "Blade Runner", "other_extra_key": None}  # OK
+    b: Movie = {
+        "name": "Blade Runner",
+        "__extra_items__": "A required regular key",
+        "other_extra_key": None,
+    }  # OK
 
 Here, ``"__extra_items__"`` in ``a`` is a regular key defined on ``Movie`` where
 its value type is narrowed from ``ReadOnly[str | None]`` to ``str``,
@@ -530,6 +534,40 @@ between this type and a closed TypedDict type::
     extra_int = not_closed  # Not OK. 'ReadOnly[object]' implicitly on 'MovieNotClosed' is not consistent with 'int' for item '__extra_items__'
     not_closed = extra_int  # OK
 
+Interaction with Constructors
+-----------------------------
+
+TypedDicts that allow extra items of type ``T`` also allow arbitrary keyword
+arguments of this type when constructed by calling the class object::
+
+    class OpenMovie(TypedDict):
+        name: str
+
+    OpenMovie(name="No Country for Old Men")  # OK
+    OpenMovie(name="No Country for Old Men", year=2007)  # Not OK. Unrecognized key
+
+    class ExtraMovie(TypedDict, closed=True):
+        name: str
+        __extra_items__: int
+
+    ExtraMovie(name="No Country for Old Men")  # OK
+    ExtraMovie(name="No Country for Old Men", year=2007)  # OK
+    ExtraMovie(
+        name="No Country for Old Men",
+        language="English",
+    )  # Not OK. Wrong type for extra key
+
+    # This implies '__extra_items__: Never',
+    # so extra keyword arguments produce an error
+    class ClosedMovie(TypedDict, closed=True):
+        name: str
+
+    ClosedMovie(name="No Country for Old Men")  # OK
+    ClosedMovie(
+        name="No Country for Old Men",
+        year=2007,
+    )  # Not OK. Extra items not allowed
+
 Interaction with Mapping[KT, VT]
 --------------------------------
 
@@ -571,14 +609,17 @@ prohibits additional required keys in its structural subtypes, we can determine
 if the TypedDict type and its structural subtypes will ever have any required
 key during static analysis.
 
-If there is no required key, the TypedDict type is consistent with ``dict[KT,
-VT]`` and vice versa if all items on the TypedDict type satisfy the following
-conditions:
+The TypedDict type is consistent with ``dict[str, VT]`` if all items on the
+TypedDict type satisfy the following conditions:
 
 - ``VT`` is consistent with the value type of the item
 
 - The value type of the item is consistent with ``VT`` 
 
+- The item is not read-only.
+
+- The item is not required.
+
 For example::
 
     class IntDict(TypedDict, closed=True):
@@ -601,6 +642,15 @@ In this case, methods that are previously unavailable on a TypedDict are allowed
 
     reveal_type(not_required_num.popitem())  # OK. Revealed type is tuple[str, int]
 
+However, ``dict[str, VT]`` is not necessarily consistent with a TypedDict type,
+because such dict can be a subtype of dict::
+
+    class CustomDict(dict[str, int]):
+        ...
+    
+    not_a_regular_dict: CustomDict = {"num": 1}
+    int_dict: IntDict = not_a_regular_dict  # Not OK
+
 How to Teach this
 =================
 
@@ -652,8 +702,8 @@ Supporting ``TypedDict(extra=type)``
 ------------------------------------
 
 While this design is potentially viable, there are several partially addressable
-concerns to consider. Adding everything up, it is slightly less favorable than
-the current proposal.
+concerns to consider. The author of this PEP thinks that it is slightly less
+favorable than the current proposal.
 
 - Usability of forward reference
   As in the functional syntax, using a quoted type or a type alias will be