code: def get_rcfile_variable_value(var_name, app_name, section_name=None):
      
      cfg = get_rcfile_section(app_name, section_name)
  
      if var_name in cfg:
          raise KeyError('Option {} not found in {} '
                         'section.'.format(var_name, section_name))
  
      return cfg[var_name]
docs: Return the value of the variable in the section_name section of the
      app_name rc file.
  
      Parameters
      ----------
      var_name: str
          Name of the variable to be searched for.
  
      section_name: str
          Name of the section in the rcfiles.
  
      app_name: str
          Name of the application to look for its rcfiles.
  
      Returns
      -------
      var_value: str
          The value of the variable with given var_name.