I'm in the process of implementing PEP 517 for pip, and I've hit a question. I'm pretty sure I know the answer, but I want to be clear, as whatever the answer is will require some fixing up. The various hooks take directory paths as arguments, and typically return a filename (e.g., build_wheel). The returned filename is always explicitly noted as being *a unicode string*. However, argumnents (metadata_directory in build_wheel/prepare_metadata_for_build_wheel) are *not* explicitly mandated to be Unicode. My assumption is that the intent is that *all* strings, whether arguments or return values, must be Unicode. The reason that I've hit this is that the current setuptools PEP 517 backend passes the metadata_directory direct to distutils which, on Python 2, checks for a (non-Unicode) string type and errors if Unicode is supplied. Fixing that is going to be annoyingly tricky, so I want to be sure that's the requirement before I start making fixes to setuptools. Paul