code: def _ModifyInterface(
        self, interface_config, config_key, config_value, replace=False):
      
      config_entry = '%s=%s' % (config_key, config_value)
      if not open(interface_config).read().count(config_key):
        with open(interface_config, 'a') as config:
          config.write('%s\n' % config_entry)
      elif replace:
        for line in fileinput.input(interface_config, inplace=True):
          print(re.sub(r'%s=.*' % config_key, config_entry, line.rstrip()))
docs: Write a value to a config file if not already present.
  
      Args:
        interface_config: string, the path to a config file.
        config_key: string, the configuration key to set.
        config_value: string, the value to set for the configuration key.
        replace: bool, replace the configuration option if already present.