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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sign_file(self, filename, signer, sign_password, keystore=None):
 """ 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. """

        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