humpty in term uses uses distlib which seems to mishandle wheel> metadata. (For example, it chokes if there's extra distribution meta and makes it impossible for buildout to install python-dateutil from a wheel.) I looked into the "mishandling". It's that the other tools don't adhere to [the current state of] PEP 426 as closely as distlib does. For example, wheel writes JSON metadata to metadata.json in the .dist-info directory, whereas PEP 426 calls for that data to be in pydist.json. The non-JSON metadata in the wheel (the METADATA file) does not strictly adhere to any of the metadata PEPs 241, 314, 345 or 426 (it has a mixture of incompatible fields). I can change distlib to look for metadata.json, and relax the rules to be more liberal regarding which fields to accept, but adhering to the PEP isn't mishandling things, as I see it. Work on distlib has slowed right down since around the time when PEP 426 was deferred indefinitely, and there seems to be little interest in progressing via metadata or other standardisation - we have to go by what the de facto tools (setuptools, wheel) choose to do. It's not an ideal situation, and incompatibilities can crop up, as you've seen. Regards, Vinay Sajip