bpo-41792: Add is_typeddict function to typing.py (GH-22254)

Closes issue41792.

Also closes https://github.com/python/typing/issues/751.

Doc/library/typing.rst

@@ -1658,6 +1658,20 @@ Introspection helpers
 
    .. versionadded:: 3.8
 
+.. function:: is_typeddict(tp)
+
+   Check if an annotation is a TypedDict class.
+
+   For example::
+        class Film(TypedDict):
+            title: str
+            year: int
+
+        is_typeddict(Film)  # => True
+        is_typeddict(Union[list, str])  # => False
+
+   .. versionadded:: 3.10
+
 .. class:: ForwardRef
 
    A class used for internal typing representation of string forward references.

Lib/test/test_typing.py

@@ -16,6 +16,7 @@
 from typing import cast, runtime_checkable
 from typing import get_type_hints
 from typing import get_origin, get_args
+from typing import is_typeddict
 from typing import no_type_check, no_type_check_decorator
 from typing import Type
 from typing import NewType
@@ -3900,6 +3901,12 @@ class Cat(Animal):
             'voice': str,
         }
 
+    def test_is_typeddict(self):
+        assert is_typeddict(Point2D) is True
+        assert is_typeddict(Union[str, int]) is False
+        # classes, not instances
+        assert is_typeddict(Point2D()) is False
+
 
 class IOTests(BaseTestCase):
 

Lib/typing.py

@@ -103,6 +103,7 @@
     'get_args',
     'get_origin',
     'get_type_hints',
+    'is_typeddict',
     'NewType',
     'no_type_check',
     'no_type_check_decorator',
@@ -1479,6 +1480,20 @@ def get_args(tp):
     return ()
 
 
+def is_typeddict(tp):
+    """Check if an annotation is a TypedDict class
+
+    For example::
+        class Film(TypedDict):
+            title: str
+            year: int
+
+        is_typeddict(Film)  # => True
+        is_typeddict(Union[list, str])  # => False
+    """
+    return isinstance(tp, _TypedDictMeta)
+
+
 def no_type_check(arg):
     """Decorator to indicate that annotations are not type hints.
 

Misc/NEWS.d/next/Library/2020-09-15-07-55-35.bpo-41792.qMpSlU.rst

@@ -0,0 +1,6 @@
+Add is_typeddict function to typing.py to check if a type is a TypedDict
+class
+
+Previously there was no way to check that without using private API. See the
+`relevant issue in python/typing
+<https://github.com/python/typing/issues/751>`