def __init__(self, source_version, target_version):
        self.source_version = source_version
        self.target_version = target_version
        self.message = (
            u"Unable to find a migration path for a version {source} "
            u"to version {target} configuration. No migration method "
            u"upgrade_from_v{source} could be found.".format(
                source=self.source_version, target=self.target_version
            )
        )
        super(MissingMigrationError, self).__init__(self.message)
Initialize a missing migration exception.
dataset/ETHPy150Open ClusterHQ/flocker/flocker/control/_persistence.py/MissingMigrationError.__init__