'check input imdbs, make sure they have same classes'

def _check_classes(self):

    try:
        self.classes = self.imdbs[0].classes
        self.num_classes = len(self.classes)
    except AttributeError:
        pass
    if (self.num_classes > 0):
        for db in self.imdbs:
            assert (self.classes == db.classes), 'Multiple    imdb    must    have    same    classes'
