You'll never pass a dictionary that doesn't have anything other than strings and numbers to tornado's finish method. Why would you use it to return things like database entries from mongo containing BSON Object IDs or anything else for that matter. And of course you won't try and coerce it to a string automatically and just fail instead.

gistfile1.py

 12 »···def finish(self, chunk=None):                                                
 13 »···»···def iterate(dictionary, comparator, mapping):                            
 14 »···»···»···for key, value in dictionary.iteritems():                            
 15 »···»···»···»···if isinstance(value, dict):                                      
 16 »···»···»···»···»···value = iterate(value, comparator, mapping)                  
 17 »···»···»···»···else:                                                            
 18 »···»···»···»···»···if comparator(value):                                        
 19 »···»···»···»···»···»···value = mapping(value)                                   
 20 »···»···»···»···dictionary[key] = value                                          
 21 »···»···»···return dictionary                                                    
 22                                                                                  
 23 »···»···def bson_check(data):                                                    
 24 »···»···»···return isinstance (data, bson.ObjectId)                              
 25                                                                                  
 26 »···»···def bson_conversion(data):                                               
 27 »···»···»···return str(data)                                                     
 28                                                                                  
 29 »···»···# Because tornado does magic...                                          
 30 »···»···if isinstance(chunk, dict):                                              
 31 »···»···»···chunk = iterate(chunk, bson_check, bson_conversion)