code: def get_credential(self, service, username):
          
          # The default implementation requires a username here.
          if username is not None:
              password = self.get_password(service, username)
              if password is not None:
                  return credentials.SimpleCredential(
                      username,
                      password,
                  )
          return None
docs: Gets the username and password for the service.
          Returns a Credential instance.
  
          The *username* argument is optional and may be omitted by
          the caller or ignored by the backend. Callers must use the
          returned username.