code: def Subclasses(cls, sort_by=None, reverse=False):
          
          l = list()
          for attr, value in get_all_attributes(cls):
              try:
                  if issubclass(value, Constant):
                      l.append((attr, value))
              except:
                  pass
  
          if sort_by is None:
              sort_by = "__creation_index__"
  
          l = list(
              sorted(l, key=lambda x: getattr(x[1], sort_by), reverse=reverse))
  
          return l
docs: Get all nested Constant class and it's name pair.
  
          :param sort_by: the attribute name used for sorting.
          :param reverse: if True, return in descend order.
          :returns: [(attr, value),...] pairs.
  
          ::
  
          >>> class MyClass(Constant):
          ...     a = 1 # non-class attributre
          ...     b = 2 # non-class attributre
          ...
          ...     class C(Constant):
          ...         pass
          ...
          ...     class D(Constant):
          ...         pass
  
          >>> MyClass.Subclasses()
          [("C", MyClass.C), ("D", MyClass.D)]
  
          .. versionadded:: 0.0.3