Could you show an example that exposes a significant overhead of the current implementation? There were reasons of using os.scandir() in os.walk() and glob.glob(). You may scan thousands of files and performs a heavy I/O only with few of them. There was smaller reason of using it in shutil.rmtree() -- removing an entry from the directory is fast, but likely you will spent much more time for creating a tree. But I don't know reasons of using it in shutil.copytree() in which you need to performs a heavy I/O with every file and directory.
To be frank, I just searched the tree for uses of listdir() combined with isdir()/isfile()/issymlink(). Thought that using scandir() would make sense, and didn't think of a reason for not using it. That being said, I cannot state a case and I'll be happy to close the PR if there's no need for this :)
The code with using scandir() is more complex and is different enough from the code with using listdir() for having significant risk of introducing bugs. Also using scandir() introduces a risk of leaking file descriptors. We don't rewrite the code without good reasons. If you propose performance enhancement, please provide benchmark results that expose the benefit of this change. If it is not large enough, it is not worth to do. Actually your change looks making the code slower: it reads the directory twice.
I believe you are right about being conservative with changing working code, specially when there's no proof about performance being improved. I think its best to close this ticket. I'd do it myself, but I know what resolution applies here.
