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Our environment variable used to be called IPYTHONDIR, at some point we changed to IPYTHON_DIR, but now we have code using both. We should settle on only one, and issue a warning when we find the alternative in use. We shouldn't flat out remove the other for a long time, as I suspect by now both are used in the wild and this would be gratuitous breakage for the users.
From an informal scan of projects, IPYTHONDIR seems to be more commonly used, but we should check more carefully. We should settle on the version that we find to be in widest use, to minimize unnecessary hassles for users.
I think the IPYTHON_DIR was added in 0.11, so it's only been around for 6 months. That form is more conventional, but I'm not sure this is something worth supporting redundancy and/or breaking historical consistency. I'd be fine with dropping the new name, and returning to the old one.
I've been scanning external projects that use IPython, and all of them use IPYTHONDIR (esp. since most were developed in the 0.10 days). While I also prefer the _ form, I do think this is unnecessary breakage of user code. I think it's ok that we break things when we absolutely must, but this one probably isn't needed.
Question: should we at least update the docs in 0.12 to mention IPYTHONDIR? I don't want to touch the actual code this late in the game, but since the code supports both, right now it's just a matter of steering our userbase in one direction or another from the documentation. If we're going to flip the order in 0.13 and add a warning, it would be good to point people towards the right option as early as possible.
I think without the underscore is fine. It also matches Python's convention, which uses PYTHONPATH, PYTHONHOME, PYTHONSTARTUP, etc.
I'm fine with IPYTHONDIR too. If anyone takes a stab at a PR, it should have:
Merge pull request #1693 from bfroehle/_1167_ipythondir
deprecate IPYTHON_DIR in favor of IPYTHONDIR
IPYTHON_DIR was a proposed rename, whose added complexity does not outweigh any potential benefit, and is reverted.