Fixed TypedDict checks causing internal TypeError on py3.8 (#219)

This works with both native and typing_extensions TypedDicts.

Fixes #218.

docs/versionhistory.rst

@@ -7,6 +7,8 @@ This library adheres to `Semantic Versioning 2.0 <https://semver.org/#semantic-v
 
 - Fixed ``TypeError`` when using typeguard within ``exec()`` (where ``__module__`` is ``None``)
   (PR by Andy Jones)
+- Fixed ``TypedDict`` causing ``TypeError: TypedDict does not support instance and class checks``
+  on Python 3.8 with standard library (not ``typing_extensions``) typed dicts
 
 **2.13.2** (2021-11-23)
 

src/typeguard/__init__.py

@@ -63,20 +63,20 @@ def isasyncgenfunction(func):
 
 if sys.version_info >= (3, 10):
     from typing import is_typeddict
-elif sys.version_info >= (3, 9):
-    def is_typeddict(tp) -> bool:
+else:
+    _typed_dict_meta_types = ()
+    if sys.version_info >= (3, 8):
         from typing import _TypedDictMeta
+        _typed_dict_meta_types += (_TypedDictMeta,)
 
-        return isinstance(tp, _TypedDictMeta)
-else:
     try:
         from typing_extensions import _TypedDictMeta
+        _typed_dict_meta_types += (_TypedDictMeta,)
     except ImportError:
-        def is_typeddict(tp) -> bool:
-            return False
-    else:
-        def is_typeddict(tp) -> bool:
-            return isinstance(tp, _TypedDictMeta)
+        pass
+
+    def is_typeddict(tp) -> bool:
+        return isinstance(tp, _typed_dict_meta_types)
 
 
 if TYPE_CHECKING:

tests/test_typeguard_py36.py

@@ -1,3 +1,4 @@
+import sys
 import warnings
 from typing import Any, AsyncGenerator, AsyncIterable, AsyncIterator, Callable, Dict
 
@@ -7,9 +8,9 @@
 from typeguard import TypeChecker, typechecked
 
 try:
-    from typing_extensions import TypedDict
-except ImportError:
     from typing import TypedDict
+except ImportError:
+    from typing_extensions import TypedDict
 
 
 @runtime_checkable
@@ -110,7 +111,9 @@ def foo(arg: ChildDict):
             pass
 
         foo({'x': 1})
-        pytest.raises(TypeError, foo, {'y': 1})
+        if sys.version_info[:2] != (3, 8):
+            # TypedDict is unusable for runtime checking on Python 3.8
+            pytest.raises(TypeError, foo, {'y': 1})
 
     def test_mapping_is_not_typeddict(self):
         """Regression test for #216."""