Add warning on abstract building subclasses

Author: parejkoj

python/style.rst

@@ -762,6 +762,27 @@ If your class may be used in multiple inheritance, ensure that all relevant clas
 
 For more details, see the `super documentation <super>`, the `astropy coding guide <http://docs.astropy.org/en/stable/development/codeguide.html#super-vs-direct-calling>`__, and `this article from Raymond Hettinger <https://rhettinger.wordpress.com/2011/05/26/super-considered-super/>`__.
 
+
+.. warning::
+    Abstract subclasses of builtins (classes that include both `abc.ABC` and a :ref:`builtin class <bltin-types>` in their hierarchy) are not checked for abstract methods on instantiation.
+    This means that `abc.abstractmethod` decorators will not prevent the class from being instantiated.
+    For example, an abstract :ref:`Exception <bltin-exceptions>` subclass can still be instantiated without a concrete override of its abstract method(s), resulting in runtime errors when those methods are called.
+    You can work around this by defining ``__new__`` on an abstract builtin subclass to check for abstract methods:
+
+    .. code-block:: python
+        :name: abstract-builtin
+
+        def __new__(cls, *args: Any, **kwargs: Any) -> MyAbstractClass:
+            # Have to override __new__ because builtin subclasses aren't checked
+            # for abstract methods; see https://github.com/python/cpython/issues/50246
+            if cls.__abstractmethods__:
+                raise TypeError(
+                    f"Can't instantiate abstract class {cls.__name__} with "
+                    f"abstract methods: {','.join(sorted(cls.__abstractmethods__))}"
+                )
+            return super().__new__(cls, *args, **kwargs)
+
+
 .. _style-guide-py-comparisons:
 
 9. Comparisons