ABCs are missing one important introspection feature. They have no API to get registered virtual subclasses. The patch implements a new method get_virtual_subclasses(recurse=False).

ABC.get_virtual_subclasses() returns the direct virtual subclasses of an ABC. ABC.get_virtual_subclasses(recuse=True) also takes subclasses of the ABC as well as subclasses of registered virtual classes into account.

Example:
>>> import numbers
>>> numbers.Number.get_virtual_subclasses()
set()
>>> numbers.Number.get_virtual_subclasses(True)
{<class 'float'>, <class 'bool'>, <class 'int'>, <class 'complex'>}
>>> numbers.Integral.get_virtual_subclasses()
{<class 'int'>}
>>> numbers.Integral.get_virtual_subclasses(True)
{<class 'int'>, <class 'bool'>}