docs: def update_constants(nmrstar2cfg="", nmrstar3cfg="", resonance_classes_cfg="", spectrum_descriptions_cfg=""):
      """Update constant variables.
  
      :return: None
      :rtype: :py:obj:`None`
      """
      nmrstar_constants = {}
      resonance_classes = {}
      spectrum_descriptions = {}
  
      this_directory = os.path.dirname(__file__)
  
      nmrstar2_config_filepath = os.path.join(this_directory, "conf/constants_nmrstar2.json")
      nmrstar3_config_filepath = os.path.join(this_directory, "conf/constants_nmrstar3.json")
      resonance_classes_config_filepath = os.path.join(this_directory, "conf/resonance_classes.json")
      spectrum_descriptions_config_filepath = os.path.join(this_directory, "conf/spectrum_descriptions.json")
  
      with open(nmrstar2_config_filepath, "r") as nmrstar2config, open(nmrstar3_config_filepath, "r") as nmrstar3config:
          nmrstar_constants["2"] = json.load(nmrstar2config)
          nmrstar_constants["3"] = json.load(nmrstar3config)
  
      with open(resonance_classes_config_filepath, "r") as config:
          resonance_classes.update(json.load(config))
  
      with open(spectrum_descriptions_config_filepath, "r") as config:
          spectrum_descriptions.update(json.load(config))
  
      if nmrstar2cfg:
          with open(nmrstar2cfg, "r") as nmrstar2config:
              nmrstar_constants["2"].update(json.load(nmrstar2config))
  
      if nmrstar3cfg:
          with open(nmrstar2cfg, "r") as nmrstar3config:
              nmrstar_constants["3"].update(json.load(nmrstar3config))
  
      if resonance_classes_cfg:
          with open(nmrstar2cfg, "r") as config:
              resonance_classes.update(json.load(config))
  
      if spectrum_descriptions_cfg:
          with open(spectrum_descriptions_cfg, "r") as config:
              spectrum_descriptions.update(json.load(config))
  
      NMRSTAR_CONSTANTS.update(nmrstar_constants)
      RESONANCE_CLASSES.update(resonance_classes)
      SPECTRUM_DESCRIPTIONS.update(spectrum_descriptions)