code: def get_first(self, attr, value, e=0.000001,
                    sort_by="__name__", reverse=False):
          
          for _, klass in self.subclasses(sort_by, reverse):
              try:
                  if getattr(klass, attr) == approx(value, e):
                      return klass
              except:
                  pass
  
          return None
docs: Get the first nested Constant class that met ``klass.attr == value``.
  
          :param attr: attribute name.
          :param value: value.
          :param e: used for float value comparison.
          :param sort_by: nested class is ordered by <sort_by> attribute.
  
          .. versionchanged:: 0.0.5