Backport NewType as it exists on py310+ (python#157)

CHANGELOG.md

@@ -66,6 +66,9 @@
 - Constructing a call-based `TypedDict` using keyword arguments for the fields
   now causes a `DeprecationWarning` to be emitted. This matches the behaviour
   of `typing.TypedDict` on 3.11 and 3.12.
+- Backport the implementation of `NewType` from 3.10 (where it is implemented
+  as a class rather than a function). This allows user-defined `NewType`s to be
+  pickled. Patch by Alex Waygood.
 
 # Release 4.5.0 (February 14, 2023)
 

README.md

@@ -181,6 +181,9 @@ Certain objects were changed after they were added to `typing`, and
   `isinstance()` checks against all these protocols were sped up significantly
   on Python 3.12. `typing_extensions` backports the faster versions to Python
   3.7+.
+- `NewType` has been in the `typing` module since Python 3.5.2, but
+  user-defined `NewType`s are only pickleable on Python 3.10+.
+  `typing_extensions.NewType` backports this feature to all Python versions.
 
 There are a few types whose interface was modified between different
 versions of typing. For example, `typing.Sequence` was modified to

src/test_typing_extensions.py

@@ -11,6 +11,7 @@
 import importlib
 import inspect
 import pickle
+import re
 import subprocess
 import tempfile
 import types
@@ -1537,23 +1538,90 @@ def foo(a: A) -> Optional[BaseException]:
 
 
 class NewTypeTests(BaseTestCase):
+    @classmethod
+    def setUpClass(cls):
+        global UserId
+        UserId = NewType('UserId', int)
+        cls.UserName = NewType(cls.__qualname__ + '.UserName', str)
+
+    @classmethod
+    def tearDownClass(cls):
+        global UserId
+        del UserId
+        del cls.UserName
 
     def test_basic(self):
-        UserId = NewType('UserId', int)
-        UserName = NewType('UserName', str)
         self.assertIsInstance(UserId(5), int)
-        self.assertIsInstance(UserName('Joe'), str)
+        self.assertIsInstance(self.UserName('Joe'), str)
         self.assertEqual(UserId(5) + 1, 6)
 
     def test_errors(self):
-        UserId = NewType('UserId', int)
-        UserName = NewType('UserName', str)
         with self.assertRaises(TypeError):
             issubclass(UserId, int)
         with self.assertRaises(TypeError):
-            class D(UserName):
+            class D(UserId):
                 pass
 
+    @skipUnless(TYPING_3_10_0, "PEP 604 has yet to be")
+    def test_or(self):
+        for cls in (int, self.UserName):
+            with self.subTest(cls=cls):
+                self.assertEqual(UserId | cls, Union[UserId, cls])
+                self.assertEqual(cls | UserId, Union[cls, UserId])
+
+                self.assertEqual(get_args(UserId | cls), (UserId, cls))
+                self.assertEqual(get_args(cls | UserId), (cls, UserId))
+
+    def test_special_attrs(self):
+        self.assertEqual(UserId.__name__, 'UserId')
+        self.assertEqual(UserId.__qualname__, 'UserId')
+        self.assertEqual(UserId.__module__, __name__)
+        self.assertEqual(UserId.__supertype__, int)
+
+        UserName = self.UserName
+        self.assertEqual(UserName.__name__, 'UserName')
+        self.assertEqual(UserName.__qualname__,
+                         self.__class__.__qualname__ + '.UserName')
+        self.assertEqual(UserName.__module__, __name__)
+        self.assertEqual(UserName.__supertype__, str)
+
+    def test_repr(self):
+        self.assertEqual(repr(UserId), f'{__name__}.UserId')
+        self.assertEqual(repr(self.UserName),
+                         f'{__name__}.{self.__class__.__qualname__}.UserName')
+
+    def test_pickle(self):
+        UserAge = NewType('UserAge', float)
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            with self.subTest(proto=proto):
+                pickled = pickle.dumps(UserId, proto)
+                loaded = pickle.loads(pickled)
+                self.assertIs(loaded, UserId)
+
+                pickled = pickle.dumps(self.UserName, proto)
+                loaded = pickle.loads(pickled)
+                self.assertIs(loaded, self.UserName)
+
+                with self.assertRaises(pickle.PicklingError):
+                    pickle.dumps(UserAge, proto)
+
+    def test_missing__name__(self):
+        code = ("import typing_extensions\n"
+                "NT = typing_extensions.NewType('NT', int)\n"
+                )
+        exec(code, {})
+
+    def test_error_message_when_subclassing(self):
+        with self.assertRaisesRegex(
+            TypeError,
+            re.escape(
+                "Cannot subclass an instance of NewType. Perhaps you were looking for: "
+                "`ProUserId = NewType('ProUserId', UserId)`"
+            )
+        ):
+            class ProUserId(UserId):
+                ...
+
 
 class Coordinate(Protocol):
     x: int
@@ -3847,7 +3915,7 @@ def test_typing_extensions_defers_when_possible(self):
         if sys.version_info < (3, 10):
             exclude |= {'get_args', 'get_origin'}
         if sys.version_info < (3, 11):
-            exclude |= {'final', 'Any'}
+            exclude |= {'final', 'Any', 'NewType'}
         if sys.version_info < (3, 12):
             exclude |= {
                 'Protocol', 'runtime_checkable', 'SupportsAbs', 'SupportsBytes',

src/typing_extensions.py

@@ -440,7 +440,6 @@ def clear_overloads():
 Counter = typing.Counter
 ChainMap = typing.ChainMap
 AsyncGenerator = typing.AsyncGenerator
-NewType = typing.NewType
 Text = typing.Text
 TYPE_CHECKING = typing.TYPE_CHECKING
 
@@ -2546,3 +2545,68 @@ class Baz(list[str]): ...
                 raise TypeError(
                     f'Expected an instance of type, not {type(__cls).__name__!r}'
                 ) from None
+
+
+# NewType is a class on Python 3.10+, making it pickleable
+# The error message for subclassing instances of NewType was improved on 3.11+
+if sys.version_info >= (3, 11):
+    NewType = typing.NewType
+else:
+    class NewType:
+        """NewType creates simple unique types with almost zero
+        runtime overhead. NewType(name, tp) is considered a subtype of tp
+        by static type checkers. At runtime, NewType(name, tp) returns
+        a dummy callable that simply returns its argument. Usage::
+            UserId = NewType('UserId', int)
+            def name_by_id(user_id: UserId) -> str:
+                ...
+            UserId('user')          # Fails type check
+            name_by_id(42)          # Fails type check
+            name_by_id(UserId(42))  # OK
+            num = UserId(5) + 1     # type: int
+        """
+
+        def __call__(self, obj):
+            return obj
+
+        def __init__(self, name, tp):
+            self.__qualname__ = name
+            if '.' in name:
+                name = name.rpartition('.')[-1]
+            self.__name__ = name
+            self.__supertype__ = tp
+            def_mod = _caller()
+            if def_mod != 'typing_extensions':
+                self.__module__ = def_mod
+
+        def __mro_entries__(self, bases):
+            # We defined __mro_entries__ to get a better error message
+            # if a user attempts to subclass a NewType instance. bpo-46170
+            supercls_name = self.__name__
+
+            class Dummy:
+                def __init_subclass__(cls):
+                    subcls_name = cls.__name__
+                    raise TypeError(
+                        f"Cannot subclass an instance of NewType. "
+                        f"Perhaps you were looking for: "
+                        f"`{subcls_name} = NewType({subcls_name!r}, {supercls_name})`"
+                    )
+
+            return (Dummy,)
+
+        def __repr__(self):
+            return f'{self.__module__}.{self.__qualname__}'
+
+        def __reduce__(self):
+            return self.__qualname__
+
+        if sys.version_info >= (3, 10):
+            # PEP 604 methods
+            # It doesn't make sense to have these methods on Python <3.10
+
+            def __or__(self, other):
+                return typing.Union[self, other]
+
+            def __ror__(self, other):
+                return typing.Union[other, self]