gh-101688: Implement types.get_original_bases

Doc/library/types.rst

@@ -82,6 +82,32 @@ Dynamic Type Creation
 
    .. versionadded:: 3.7
 
+.. function:: get_original_bases(cls, /)
+
+    Return the objects in the bases list of the class's definition as they
+    existed prior to any modification by :meth:`~object.__mro_entries__`. This
+    is useful for introspecting :ref:`Generics <user-defined-generics>`.
+
+    Examples::
+
+        from typing import TypeVar, Generic
+
+        T = TypeVar("T")
+        class Foo(Generic[T]): ...
+        class Bar(Foo[int], float): ...
+        class Baz(list[str]): ...
+
+        assert Bar.__bases__ == (Foo, float)
+        assert get_original_bases(Bar) == (Foo[int], float)
+
+        assert Baz.__bases__ == (list,)
+        assert get_original_bases(Baz) == (list[str],)
+
+        assert int.__bases__ == (object,)
+        assert get_original_bases(int) == (object,)
+
+    .. versionadded:: 3.12
+
 .. seealso::
 
    :pep:`560` - Core support for typing module and generic types

Doc/reference/datamodel.rst

@@ -2097,11 +2097,21 @@ Resolving MRO entries
    of classes that will be used instead of the base. The returned tuple may be
    empty: in these cases, the original base is ignored.
 
+
+.. attribute:: type.__orig_bases__
+
+   If the class originally had bases modified by :meth:`~object.__mro_entries__`,
+   this the *bases* tuple.
+
 .. seealso::
 
    :func:`types.resolve_bases`
       Dynamically resolve bases that are not instances of :class:`type`.
 
+   :func:`types.get_original_bases`
+      Retrieve a class's "original bases" prior to modifications by
+      :meth:`~object.__mro_entries__`.
+
    :pep:`560`
       Core support for typing module and generic types.
 

Doc/whatsnew/3.12.rst

@@ -392,6 +392,13 @@ threading
   profiling functions in all running threads in addition to the calling one.
   (Contributed by Pablo Galindo in :gh:`93503`.)
 
+types
+-----
+
+* Add :func:`types.get_original_bases` to allow for further introspection of
+  :ref:`user-defined-generics` when subclassed. (Contributed by
+  James Hilton-Balfe and Alex Waygood in :gh:`101827`.)
+
 unicodedata
 -----------
 

Lib/test/test_types.py

@@ -1360,6 +1360,32 @@ class C: pass
         D = types.new_class('D', (A(), C, B()), {})
         self.assertEqual(D.__bases__, (A1, A2, A3, C, B1, B2))
 
+    def test_get_original_bases(self):
+        T = typing.TypeVar('T')
+        class A: pass
+        class B(typing.Generic[T]): pass
+        class C(B[int]): pass
+        class D(B[str], float): pass
+        self.assertEqual(types.get_original_bases(A), (object,))
+        self.assertEqual(types.get_original_bases(B), (typing.Generic[T],))
+        self.assertEqual(types.get_original_bases(C), (B[int],))
+        self.assertEqual(types.get_original_bases(int), (object,))
+        self.assertEqual(types.get_original_bases(D), (B[str], float))
+
+        class E(list[T]): pass
+        class F(list[int]): pass
+
+        self.assertEqual(types.get_original_bases(E), (list[T],))
+        self.assertEqual(types.get_original_bases(F), (list[int],))
+
+        class G(typing.NamedTuple):
+            x: int
+
+        self.assertIs(types.get_original_bases(G)[0], typing.NamedTuple)
+
+        with self.assertRaisesRegex(TypeError, "Expected an instance of type"):
+            types.get_original_bases(object())
+
     # Many of the following tests are derived from test_descr.py
     def test_prepare_class(self):
         # Basic test of metaclass derivation

Lib/types.py

@@ -143,6 +143,34 @@ 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
+
+        T = TypeVar("T")
+        class Foo(Generic[T]): ...
+        class Bar(Foo[int], float): ...
+        class Baz(list[str]): ...
+
+        assert get_original_bases(Bar) == (Foo[int], float)
+        assert get_original_bases(Baz) == (list[str],)
+        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__.
 

Misc/NEWS.d/next/Library/2023-02-11-15-01-32.gh-issue-101688.kwXmfM.rst

@@ -0,0 +1,2 @@
+Implement :func:`types.get_original_bases` to provide further introspection
+for types.