gh-62912: fix pure-Python operator.index()

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: skirpichev

Doc/library/operator.rst

@@ -122,7 +122,7 @@ The mathematical and bitwise operations are the most numerous:
 .. function:: index(a)
               __index__(a)
 
-   Return *a* converted to an integer.  Equivalent to ``a.__index__()``.
+   Return *a* converted to an integer.  Equivalent to ``int(type(a).__index__(a))``.
 
    .. versionchanged:: 3.10
       The result always has exact type :class:`int`.  Previously, the result

Lib/operator.py

@@ -93,8 +93,11 @@ def floordiv(a, b):
     return a // b
 
 def index(a):
-    "Same as a.__index__()."
-    return a.__index__()
+    "Same as int(type(a).__index__(a))."
+    if hasattr(type(a), '__index__'):
+        return int(a)
+    raise TypeError(f"'{type(a).__name__}' object cannot be "
+                    "interpreted as an integer")
 
 def inv(a):
     "Same as ~a."

Lib/test/test_operator.py

@@ -570,6 +570,34 @@ def __index__(self):
         with self.assertRaises((AttributeError, TypeError)):
             operator.index(None)
 
+        self.assertRaises(TypeError, operator.index)
+        self.assertRaises(TypeError, operator.index, 1, 1)
+        self.assertEqual(operator.index(True), 1)
+        self.assertRaises(TypeError, operator.index, '42')
+        self.assertRaises(TypeError, operator.index, 42.0)
+        x = X()
+        x.__index__ = lambda: 1729
+        self.assertEqual(operator.index(x), 1)
+        class F:
+            def __index__(self):
+                return 42.0
+        self.assertRaises(TypeError, operator.index, F())
+        class I(int):
+            def __index__():
+                return 42
+        i = I()
+        self.assertIs(operator.index(i), int(i))
+        class D:
+            def __index__(self):
+                return I(42)
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(operator.index(D()), 42)
+        class S:
+            @staticmethod
+            def __index__():
+                return 42
+        self.assertEqual(operator.index(S()), 42)
+
     def test_not_(self):
         operator = self.module
         class C:

Misc/NEWS.d/next/Library/2026-04-03-01-53-16.gh-issue-62912.DHA_yk.rst

@@ -0,0 +1,2 @@
+Correct pure-Python version of the :func:`operator.index` to behave like the
+C version.  Patch by Sergey B Kirpichev.