Backport assert_never change to include repr of value (#371)

Author: Hnasar

CHANGELOG.md

@@ -4,6 +4,9 @@
   `__static_attributes__` attribute to all classes in Python,
   which broke some assumptions made by the implementation of
   `typing_extensions.Protocol`.
+- At runtime, `assert_never` now includes the repr of the argument
+  in the `AssertionError`. Patch by Hashem, backporting of the original
+  fix https://github.com/python/cpython/pull/91720 by Jelle Zijlstra.
 
 # Release 4.11.0 (April 5, 2024)
 

src/test_typing_extensions.py

@@ -283,6 +283,19 @@ def test_exception(self):
         with self.assertRaises(AssertionError):
             assert_never(None)
 
+        value = "some value"
+        with self.assertRaisesRegex(AssertionError, value):
+            assert_never(value)
+
+        # Make sure a huge value doesn't get printed in its entirety
+        huge_value = "a" * 10000
+        with self.assertRaises(AssertionError) as cm:
+            assert_never(huge_value)
+        self.assertLess(
+            len(cm.exception.args[0]),
+            typing_extensions._ASSERT_NEVER_REPR_MAX_LENGTH * 2,
+        )
+
 
 class OverrideTests(BaseTestCase):
     def test_override(self):

src/typing_extensions.py

@@ -2355,6 +2355,12 @@ def reveal_type(obj: T, /) -> T:
         return obj
 
 
+if hasattr(typing, "_ASSERT_NEVER_REPR_MAX_LENGTH"):  # 3.11+
+    _ASSERT_NEVER_REPR_MAX_LENGTH = typing._ASSERT_NEVER_REPR_MAX_LENGTH
+else:  # <=3.10
+    _ASSERT_NEVER_REPR_MAX_LENGTH = 100
+
+
 if hasattr(typing, "assert_never"):  # 3.11+
     assert_never = typing.assert_never
 else:  # <=3.10
@@ -2378,7 +2384,10 @@ def int_or_str(arg: int | str) -> None:
         At runtime, this throws an exception when called.
 
         """
-        raise AssertionError("Expected code to be unreachable")
+        value = repr(arg)
+        if len(value) > _ASSERT_NEVER_REPR_MAX_LENGTH:
+            value = value[:_ASSERT_NEVER_REPR_MAX_LENGTH] + '...'
+        raise AssertionError(f"Expected code to be unreachable, but got: {value}")
 
 
 if sys.version_info >= (3, 12):  # 3.12+