Add Nix environment definition

[kierdavis]

This enables Nix users to simply run nix-shell in the repository root to obtain a shell with python and pipenv (and anything else we may want to add in the future) available.

[mildlyincompetent]

I recall commenting on a similar PR that this is nasty and should not go in the repo, however as I am now a NixOS user, I think this is a great idea and should go in all repos.

[trickeydan]

This works and will be useful for when I switch to NixOS.

[trickeydan]

I recall commenting on a similar PR that this is nasty and should not go in the repo, however as I am now a NixOS user, I think this is a great idea and should go in all repos.

Well I'm strongly considering building the new Pi OS on nix, so this is very useful.

[mildlyincompetent]

Huh, on my system the python3 requirement was satisfied using python 3.6, which doesn't work as we require 3.7. Is it possible to specify this more precisely?

[kierdavis]

@mildlyincompetent it is possible (python3 -> python37).

I thought we were still compatible with 3.6? We really need to make a decision on #79.

[mildlyincompetent]

The pip / pipenv config isn't, so the repo isn't 3.6 compatible currently, though this may be subject to change once we work out what exactly we're supporting.

[kierdavis]

Regardless, all evidence suggests that 3.7 is the preferred version for development, so I'll specify this in the shell.nix.

[kierdavis]

Ugh. As a result of Nix's default python3 version being 3.6 (on the 18.09 release, haven't checked unstable), one must go out of their way to override python3Packages = python37Packages. Hydra doesn't appear to build python packages against any python version other than the default, so using a non-default python version here forces pipenv and its dependencies to be built locally. Luckily this is all automated but its a bit annoying for nix-shell to now take 10 minutes to set the environment up.

[mildlyincompetent]

Hey at least we're not attempting to emerge, could be a worse distro. Does this process only take forever once (since I assume its results live in the nix store), or does it take ages every time? If its the former I would say it is grudgingly acceptable.

Unstable uses 3.7 I believe judging by the random docker container Dan just downloaded, and appears to have binaries for 3.7.

[kierdavis]

Yes, results will persist in the store, so it'll only take forever once (unless you garbage collect the store automatically, in which case it'll take forever once per week).

[kierdavis]

Thinking about it the point about nix-shell taking a long time is moot. pipenv is broken on the current stable channel, so one must use unstable in the meantime. The next release will almost certainly make python 3.7 the default.

@trickeydan please can you check my recent commits haven't broken this for you?

[trickeydan]

Yep, still works for me.