def previous(self):
        index = self.migrations.index(self) - 1
        if index < 0:
            return None
        return self.migrations[index]
parse filters set from outside to standard format.
dataset/ETHPy150Open cloudera/hue/desktop/core/ext-py/South-1.0.2/south/migration/base.py/Migration.previous--crossover with--dataset/ETHPy150Open openstack/compass-core/compass/db/models.py/Switch.parse_filters