code: def get_value(self, section, option,
                    default=None):
          
          ret = default
          try:
              ret = self.parser.get(section, option)
          except NoOptionError:
              pass
  
          # Search a substring `foo` and replace it by the result of its exec
          if ret is not None:
              try:
                  match = self.re_pattern.findall(ret)
                  for m in match:
                      ret = ret.replace(m, system_exec(m[1:-1]))
              except TypeError:
                  pass
          return ret
docs: Get the value of an option, if it exists.
  
          If it did not exist, then return the default value.
  
          It allows user to define dynamic configuration key (see issue#1204)
          Dynamic vlaue should starts and end with the ` char
          Example: prefix=`hostname`