human
Solve the following python programming problem given the following code:
def apply(self, fgraph) : 
    tasks = defaultdict (list) 
    if (self.max_use_ratio is not None) : 
        max_uses = (self.max_use_ratio * len (fgraph.apply_nodes)) 
        runs = defaultdict (int) 
else : 
        runs = None 
def importer(node) : 
        self.backtrack (node, tasks) 
def pruner(node) : 
        try : 
            del tasks [node] 
except KeyError : 
            pass 
def chin(node, i, r, new_r) : 
        if (new_r.owner and (not r.clients)) : 
            self.backtrack (new_r.owner, tasks) 
for node in fgraph.toposort () : 
        tasks [node].extend ((lopt for (track, i, lopt) in self.fetch_tracks0 (node.op))) 
u = self.attach_updater (fgraph, pruner, pruner, chin) 
    print ('KEYS', [hash (t) for t in tasks.keys ()]) 
    while tasks : 
        for node in tasks : 
            todo = tasks.pop (node) 
            break 
for lopt in todo : 
            if ((runs is not None) and (runs [lopt] >= max_uses)) : 
                print (('Warning: optimization exceeded its maximal use ratio: %s, %s' % (lopt, max_uses)), file = sys.stderr) 
                continue 
success = self.process_node (fgraph, node, lopt) 
            if success : 
                if (runs is not None) : 
                    runs [lopt] += 1 
break 
self.detach_updater (fgraph, u) 

    
    

    Fix the buggy line: u = self.attach_updater (fgraph, pruner, pruner, chin)
assistant
#Here is Python program to fix the buggy line: u = self.

u = self.attach_updater (fgraph, def importer(, pruner, chin)