'Initialize a missing migration exception.
:param int source_version: The version to migrate from.
:param int target_version: The version to migrate to.'

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}    to    version    {target}    configuration.    No    migration    method    upgrade_from_v{source}    could    be    found.'.format(source=self.source_version, target=self.target_version)
    super(MissingMigrationError, self).__init__(self.message)
