class_docstr: A string holding a number format specifier, e.g. '.3f'
  
  This traitlet holds a string that can be passed to the
  `d3-format <https://github.com/d3/d3-format>`_ JavaScript library.
  The format allowed is similar to the Python format specifier (PEP 3101).
code: def validate(self, obj, value):
      value = super().validate(obj, value)
      re_match = _number_format_re.match(value)
      if re_match is None:
          self.error(obj, value)
      else:
          format_type = re_match.group(9)
          if format_type is None:
              return value
          elif format_type in _number_format_types:
              return value
          else:
              raise traitlets.TraitError(
                  "The type specifier of a NumberFormat trait must "
                  "be one of {}, but a value of '{}' was "
                  "specified.".format(
                      list(_number_format_types), format_type
                  )
              )
  
imports: import re
  import traitlets
  import datetime
label_desc: Use image node_id 1 for calling the NumberFormat obj's underlying member method code with example usage: obj.validate(obj, value) and returns: value, value