code: def load_class(cls, identifier, default=None, select=None):
          
          identifier = identifier.lower()
          key = (cls.entry_point, identifier)
          if key not in PLUGIN_CACHE:
  
              if select is None:
                  select = default_select
  
              all_entry_points = list(pkg_resources.iter_entry_points(cls.entry_point, name=identifier))
              for extra_identifier, extra_entry_point in cls.extra_entry_points:
                  if identifier == extra_identifier:
                      all_entry_points.append(extra_entry_point)
  
              try:
                  selected_entry_point = select(identifier, all_entry_points)
              except PluginMissingError:
                  if default is not None:
                      return default
                  raise
  
              PLUGIN_CACHE[key] = cls._load_class_entry_point(selected_entry_point)
  
          return PLUGIN_CACHE[key]
docs: Load a single class specified by identifier.
  
          If `identifier` specifies more than a single class, and `select` is not None,
          then call `select` on the list of entry_points. Otherwise, choose
          the first one and log a warning.
  
          If `default` is provided, return it if no entry_point matching
          `identifier` is found. Otherwise, will raise a PluginMissingError
  
          If `select` is provided, it should be a callable of the form::
  
              def select(identifier, all_entry_points):
                  # ...
                  return an_entry_point
  
          The `all_entry_points` argument will be a list of all entry_points matching `identifier`
          that were found, and `select` should return one of those entry_points to be
          loaded. `select` should raise `PluginMissingError` if no plugin is found, or `AmbiguousPluginError`
          if too many plugins are found