def on_attach(self, fgraph):

        # Do the checking #
        already_there = False
        if self.fgraph is fgraph:
            already_there = True
        if self.fgraph is not None:
            raise Exception(
                "A DestroyHandler instance can only serve one"
                " FunctionGraph. (Matthew 6:24)")
        for attr in ('destroyers', 'destroy_handler'):
            if hasattr(fgraph, attr):
                already_there = True

        if already_there:
            # FunctionGraph.attach_feature catches AlreadyThere and cancels the attachment
            raise toolbox.AlreadyThere(
                "DestroyHandler feature is already present"
                " or in conflict with another plugin.")

        # Annotate the FunctionGraph #
        self.unpickle(fgraph)
        fgraph.destroy_handler = self

        self.fgraph = fgraph
        self.destroyers = OrderedSet()  # set of Apply instances with non-null destroy_map
        self.view_i = OrderedDict()  # variable -> variable used in calculation
        self.view_o = OrderedDict()  # variable -> set of variables that use this one as a direct input
        # clients: how many times does an apply use a given variable
        self.clients = OrderedDict()  # variable -> apply -> ninputs
        self.stale_droot = True

        self.debug_all_apps = OrderedSet()
        if self.do_imports_on_attach:
            toolbox.Bookkeeper.on_attach(self, fgraph)
Tests that it chooses the best strategy.
dataset/ETHPy150Open rizar/attention-lvcsr/libs/Theano/theano/gof/destroyhandler.py/DestroyHandler.on_attach--crossover with--dataset/ETHPy150Open Axelrod-Python/Axelrod/axelrod/tests/unit/test_qlearner.py/TestCautiousQLearner.test_strategy