gh-105570: Deprecate unusual ways of creating empty TypedDicts (#105780)

Deprecate two methods of creating typing.TypedDict classes with 0 fields using the functional syntax: `TD = TypedDict("TD")` and `TD = TypedDict("TD", None)`. Both will be disallowed in Python 3.15. To create a TypedDict class with 0 fields, either use `class TD(TypedDict): pass` or `TD = TypedDict("TD", {})`.

1 files changed, 17 insertions, 2 deletions

diff --git a/Lib/typing.py b/Lib/typing.py
index 570cb80..1dd9398 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -2908,7 +2908,7 @@ class _TypedDictMeta(type):
     __instancecheck__ = __subclasscheck__
 
 
-def TypedDict(typename, fields=None, /, *, total=True):
+def TypedDict(typename, fields=_sentinel, /, *, total=True):
     """A simple typed namespace. At runtime it is equivalent to a plain dict.
 
     TypedDict creates a dictionary type such that a type checker will expect all
@@ -2955,7 +2955,22 @@ def TypedDict(typename, fields=None, /, *, total=True):
 
     See PEP 655 for more details on Required and NotRequired.
     """
-    if fields is None:
+    if fields is _sentinel or fields is None:
+        import warnings
+
+        if fields is _sentinel:
+            deprecated_thing = "Failing to pass a value for the 'fields' parameter"
+        else:
+            deprecated_thing = "Passing `None` as the 'fields' parameter"
+
+        example = f"`{typename} = TypedDict({typename!r}, {{{{}}}})`"
+        deprecation_msg = (
+            "{name} is deprecated and will be disallowed in Python {remove}. "
+            "To create a TypedDict class with 0 fields "
+            "using the functional syntax, "
+            "pass an empty dictionary, e.g. "
+        ) + example + "."
+        warnings._deprecated(deprecated_thing, message=deprecation_msg, remove=(3, 15))
         fields = {}
 
     ns = {'__annotations__': dict(fields)}