[语法糖] 关于类别字符串到类别数字id的映射

在https://github.com/ming71/DAL，dota_dataset.py 中：

self.classes = ('__background__', 'plane', 'ship', 'storage-tank', 'baseball-diamond', 
                'tennis-court', 'basketball-court', 'ground-track-field', 'harbor', 
                'bridge', 'large-vehicle', 'small-vehicle', 'helicopter', 'roundabout', 
                'soccer-ball-field' , 'swimming-pool')   
self.num_classes = len(self.classes)
self.class_to_ind = dict(zip(self.classes, range(self.num_classes)))   

类别到数字id的转换只用了一行，妙啊，zip和dict还能这种配合