[Catalog-sig] Flag to tell pip to only install uploaded files

[martin at v.loewis.de]

> How would this flag be presented to pip? How would a package
> maintainer manage it?

One way of presenting it would be a separate index URL, such
as pypi.python.org/unstable/. Releases would have to be marked
as unstable during upload, and then appear only in the unstable
index (and in the owner's release management page).

Tools that download wouldn't need any extra configuration except
being pointed to the alternate index.

Regards,
Martin