code: def _header(self, instance):
      cells = []
      cells.append(self._format_key("iter"))
      cells.append(self._format_key("target"))
  
      if self._is_constrained:
          cells.append(self._format_key("allowed"))
  
      for key in instance.space.keys:
          cells.append(self._format_key(key))
  
      line = "| " + " | ".join(cells) + " |"
      self._header_length = len(line)
      return line + "\n" + ("-" * self._header_length)
  
imports: from __future__ import print_function
  import os
  import json
  import warnings
  from pathlib import Path
  from .observer import _Tracker
  from .event import Events
  from .util import Colours
label_desc: Use image node_id 9 for calling the ScreenLogger obj's underlying member method code with example usage: obj._header(instance) and returns: unknown