code: def parse(self, key, value):
          
          if value is not None:
              try:
                  return self._parser(value)
              except Exception:
                  raise ParsingError("Error parsing {}".format(key))
          elif self._default is not SENTINAL:
              return self._default
          else:
              raise KeyError(key)
docs: Parse the environment value for a given key against the schema.
  
          Args:
              key: The name of the environment variable.
              value: The value to be parsed.