PIG 3 - Plan for dropping Python 2.7 support

[cdeil]

This PIG is a proposal that we should drop Python 2.7 support with Gammapy and users and developers are encouraged to update to Python 3 now. No timescale is proposed yet, this is just to mention the goal and have a place for discussion on this question.

• Let us know if you need help to update to Python 3!
• Let us know if you can't update for some reason, and need continued Python 2.7 support from future Gammapy releases.

cc @woodmd and @eacharles from Fermipy.

[woodmd]

The Fermi STs only support python 2 so for the moment Fermipy can't update to Python 3 (although some modules that don't import the STs have already been updated). To the extent that we would like to continue moving functionality from Fermipy into gammapy we would prefer if gammapy could keep python 2 support until we can update Fermipy to Python 3.

I've recently had some discussions within the Fermi team and there seems to be support for migrating the Fermi STs to python 3. However the timeline is still pretty uncertain and I doubt a python 3 compatible ST release would be ready sooner than a ~year from now. We're having our software meeting the week after next so we will have some more information about timelines at that point.

[cdeil]

@woodmd or @dimauromattia or @eacharles - is there a plan / timeline for Python 3 support in the Fermi ST and Fermipy?

A recent CTA Gammapy user survey (50 responses) showed that 40% still use Python 2.7, which I found surprising. I thought the fraction of Python 2 users very small already.

So overall it looks like we'll keep supporting Python 2.7 for a while. But given that many packages (e.g. Jupyter / ipython / Astropy) have dropped Python 2.7 support and a more (e.g. Numpy and core Python itself) will drop it in 2019. It would be nice if we in Gammapy could as well at some point.

There's also https://github.com/astropy/astropy-healpix and https://github.com/astropy/regions which very likely will mature and land as astropy.healpix and astropy.regions in Astropy core, and since Astropy is Python 3 only already, there's a pretty good motivation to drop Python 2 support in those packages now (which simplifies the C interfaces e.g.), exept for that fact that we still need to support Python 2 in Gammapy and are using regions already and want to use astropy-healpix very soon.