def __init__(self, handler):
        self.handler = handler

    # called when a mutator (eg. "has_field(...)") is parsed
check if db exists.
dataset/ETHPy150Open jmg/elixir/elixir/statements.py/ClassMutator.__init__--crossover with--dataset/ETHPy150Open openstack/compass-core/bin/manage_db.py/checkdb