Reimplement TypedDict in a similar way to Python (#191)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Unreleased
 
+- Align the implementation of `TypedDict` with the implementation in the
+  standard library on Python 3.9 and higher.
+  `typing_extensions.TypedDict` is now a function instead of a class. The
+  private functions `_check_fails`, `_dict_new`, and `_typeddict_new`
+  have been removed. `is_typeddict` now returns `False` when called with
+  `TypedDict` itself as the argument. Patch by Jelle Zijlstra.
 - Declare support for Python 3.12. Patch by Jelle Zijlstra.
 - Fix tests on Python 3.13, which removes support for creating
   `TypedDict` classes through the keyword-argument syntax. Patch by

doc/index.rst

@@ -347,6 +347,12 @@ Special typing primitives
 
       Support for the ``__orig_bases__`` attribute was added.
 
+   .. versionchanged:: 4.7.0
+
+      ``TypedDict`` is now a function rather than a class.
+      This brings ``typing_extensions.TypedDict`` closer to the implementation
+      of :py:mod:`typing.TypedDict` on Python 3.9 and higher.
+
 .. class:: TypeVar(name, *constraints, bound=None, covariant=False,
                    contravariant=False, infer_variance=False, default=...)
 
@@ -680,6 +686,12 @@ Functions
 
    .. versionadded:: 4.1.0
 
+   .. versionchanged:: 4.7.0
+
+      :func:`is_typeddict` now returns ``False`` when called with
+      :data:`TypedDict` itself as the argument, consistent with the
+      behavior of :py:func:`typing.is_typeddict`.
+
 .. function:: reveal_type(obj)
 
    See :py:func:`typing.reveal_type`. In ``typing`` since 3.11.

src/_typed_dict_test_helper.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from typing import Generic, Optional, T
-from typing_extensions import TypedDict
+from typing_extensions import TypedDict, Annotated, Required
 
 
 # this class must not be imported into test_typing_extensions.py at top level, otherwise
@@ -16,3 +16,7 @@ class Foo(TypedDict):
 
 class FooGeneric(TypedDict, Generic[T]):
     a: Optional[T]
+
+
+class VeryAnnotated(TypedDict, total=False):
+    a: Annotated[Annotated[Annotated[Required[int], "a"], "b"], "c"]