code: def _check_classes(self):
          
          try:
              self.classes = self.imdbs[0].classes
              self.num_classes = len(self.classes)
          except AttributeError:
              # fine, if no classes is provided
              pass
  
          if self.num_classes > 0:
              for db in self.imdbs:
                  assert self.classes == db.classes, "Multiple imdb must have same classes"
docs: check input imdbs, make sure they have same classes