docs: def get_class(classname, all=False):
      """Retrieve a class from the registry.
  
      :raises: marshmallow.exceptions.RegistryError if the class cannot be found
          or if there are multiple entries for the given class name.
      """
      try:
          classes = _registry[classname]
      except KeyError:
          raise RegistryError('Class with name {!r} was not found. You may need '
              'to import the class.'.format(classname))
      if len(classes) > 1:
          if all:
              return _registry[classname]
          raise RegistryError('Multiple classes with name {!r} '
              'were found. Please use the full, '
              'module-qualified path.'.format(classname))
      else:
          return _registry[classname][0]