gh-101688: Implement types.get_original_bases (#101827)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/Lib/types.py b/Lib/types.py
index aa8a1c8..6110e6e 100644
--- a/Lib/types.py
+++ b/Lib/types.py
@@ -143,6 +143,38 @@ def _calculate_meta(meta, bases):
                         "of the metaclasses of all its bases")
     return winner
 
+
+def get_original_bases(cls, /):
+    """Return the class's "original" bases prior to modification by `__mro_entries__`.
+
+    Examples::
+
+        from typing import TypeVar, Generic, NamedTuple, TypedDict
+
+        T = TypeVar("T")
+        class Foo(Generic[T]): ...
+        class Bar(Foo[int], float): ...
+        class Baz(list[str]): ...
+        Eggs = NamedTuple("Eggs", [("a", int), ("b", str)])
+        Spam = TypedDict("Spam", {"a": int, "b": str})
+
+        assert get_original_bases(Bar) == (Foo[int], float)
+        assert get_original_bases(Baz) == (list[str],)
+        assert get_original_bases(Eggs) == (NamedTuple,)
+        assert get_original_bases(Spam) == (TypedDict,)
+        assert get_original_bases(int) == (object,)
+    """
+    try:
+        return cls.__orig_bases__
+    except AttributeError:
+        try:
+            return cls.__bases__
+        except AttributeError:
+            raise TypeError(
+                f'Expected an instance of type, not {type(cls).__name__!r}'
+            ) from None
+
+
 class DynamicClassAttribute:
     """Route attribute access on a class to __getattr__.