[Nix-dev] Python 2 versus 3

[Andres Loeh]

Probably, renaming to python3 is the most pragmatic option available.

But in general, I think this is due to the actually rather
short-sighted assumption that upgrades are desired from a package
pkg-m to a package pkg-n if an only if m<n.

This is, in general, not true. The python situation is a case where
installing python-3 over python-2 automatically is almost never
desired. However, there are other situations where the assumption is
incorrect as well, the most frequent being renames of packages, but
also random changes in the version numbering scheme. In such cases, we
might suddenly want to upgrade from pkg-17.3 to gkp-4.5.

It would be absolutely amazing if the Nix expression language would
just let us specify how the directed set of version upgrades should
look like. I.e., we'd need a way to specify that a package constitutes
an upgrade for another even if the names are unrelated.

Just dreaming ...

Cheers,
  Andres