class_docstr: Define the traitlet interface with `PerspectiveJupyterWidget` on the
  front end. Attributes which are set here are synchronized between the
  front-end and back-end.
  
  Examples:
      >>> widget = perspective.PerspectiveWidget(
      ...     data, group_by=["a", "b", "c"])
      PerspectiveWidget(group_by=["a", "b", "c"])
      >>> widget.split_by=["b"]
      >>> widget
      PerspectiveWidget(group_by=["a", "b", "c"], split_by=["b"])
code: def _validate_sort(self, proposal):
      return validate_sort(proposal.value)
  
file_path: perspective/python/perspective/perspective/viewer/viewer_traitlets.py
imports: from traitlets import HasTraits, Unicode, List, Bool, Dict, validate
  from ..core._version import __version__
  from .validate import validate_plugin, validate_columns, validate_group_by, validate_split_by, validate_aggregates, validate_sort, validate_filter, validate_expressions, validate_plugin_config, validate_title, validate_version
label_desc: Use image node_id 6 for calling the PerspectiveTraitlets obj's underlying member method code with example usage: obj._validate_sort(proposal) and returns: validate_sort