code: def sign_file(self, filename, signer, sign_password, keystore=None):
          
          cmd, sig_file = self.get_sign_command(filename, signer, sign_password,
                                                keystore)
          rc, stdout, stderr = self.run_command(cmd,
                                                sign_password.encode('utf-8'))
          if rc != 0:
              raise DistlibException('sign command failed with error '
                                     'code %s' % rc)
          return sig_file
docs: Sign a file.
  
          :param filename: The pathname to the file to be signed.
          :param signer: The identifier of the signer of the file.
          :param sign_password: The passphrase for the signer's
                                private key used for signing.
          :param keystore: The path to a directory which contains the keys
                           used in signing. If not specified, the instance's
                           ``gpg_home`` attribute is used instead.
          :return: The absolute pathname of the file where the signature is
                   stored.