I try to read faster r-cnn 7 implemented by ruotianluo. Almost all important python files are in ‘‘lib’’  fold, which is not a package (there is no init.py file under it). I really don’t understand why modules can be import using code like ‘from model.config import cfg’ (model is a fold in lib, and config is a ‘.py’ file under model).
The detailed program structure can be found in https://github.com/ruotianluo/pytorch-faster-rcnn 7, and the import sample can be seen in https://github.com/ruotianluo/pytorch-faster-rcnn/blob/master/lib/model/train_val.py 4