List all registered classes from `abc.ABC.register`

Ideas
1

Currently, as far as I know, there’s no way to recover the registered subclasses of abc.ABC (through cls.register(MyClass))

class A:
  pass

class Array(abc.ABC):
  pass


Array.register(A)  # How to recover A from Array after this call ?

assert isinstance(A(), Array)

abc.get_registered(Array)  # == [A]   # <<< How to do this ?

It seems that Array has a Array._abc_impl but I wasn’t able to recover the list of registered subclasses from it.

Example of use-case:

IPython allow to register some custom rendering function for specific classes:

ipython = IPython.get_ipython()
formatter = ipython.display_formatter.formatters['text/html']
formatter.for_type(Array, _display_array)  # Save `_cls_to_format_fn = {Array: _display_array}`

However because Array is not in the A.mro(), displaying A() won’t call our custom display function.
Indeed IPython matching is done through lookup (cls in {Array: _display_array}) and not isinstance.

With abc.get_registered(Array), it would be possible to register all subclasses:

for cls in abc.get_registered(Array):
  formatter.for_type(cls, _display_array)
2

Interesting use case, but isn’t a better solution to this use case for iPython to make it so that registering the base class is enough for all subclasses to use that code path? This is how single dispatch registration works.