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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 verify_signature(self, signature_filename, data_filename, keystore=None):
 """ Verify a signature for a file. :param signature_filename: The pathname to the file containing the signature. :param data_filename: The pathname to the file containing the signed data. :param keystore: The path to a directory which contains the keys used in verification. If not specified, the instance's ``gpg_home`` attribute is used instead. :return: True if the signature was verified, else False. """

        if not self.gpg:
            raise DistlibException('verification unavailable because gpg '
                                   'unavailable')
        cmd = self.get_verify_command(signature_filename, data_filename,
                                      keystore)
        rc, stdout, stderr = self.run_command(cmd)
        if rc not in (0, 1):
            raise DistlibException('verify command failed with error '
                             'code %s' % rc)
        return rc == 0