code: def GetCredential(self, path_spec, identifier):
      
      credentials = self._credentials_per_path_spec.get(path_spec.comparable, {})
      return credentials.get(identifier, None)
docs: Retrieves a specific credential from the key chain.
  
      Args:
        path_spec (PathSpec): path specification.
        identifier (str): credential identifier.
  
      Returns:
        object: credential or None if the credential for the path specification
            is not set.