remove Python 2 constructs

[orbeckst]

Release 2.0 does not support Python 2 anymore (see the Roadmap) so we should get rid of compatibility code (six and friends).

EDIT: In this issue we also decided to drop Python 3.5 support from 2.0.

[RMeli]

Which versions of Python 3 will be supported? (Asking mainly for f-strings, PEP 498)

[orbeckst]

I don’t think we discussed this in any detail. Start a discussion on the dev list?

…

Am 19.02.2020 um 04:05 schrieb Rocco Meli ***@***.***>:  Which versions of Python 3 will be supported? (Asking mainly for f-strings, PEP 498) — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or unsubscribe.

[richardjgowers]

I think we usually followed whatever numpy was also doing, which I think was what continuous integration and conda-forge supported. I think we usually have to be quite slow because we need to support that one cluster that still happens to run 2.7 in 2020.

[IAlibay]

I don't know when 1.19 comes out, but numpy seems to have dropped support for 3.5 on its development branch: https://github.com/numpy/numpy/blob/master/INSTALL.rst.txt#prerequisites

My understanding is that major releases of numpy are scheduled ~ 6 months, which means 1.19 should come in late June. So that might align itself quite well with a late Q2 goal for MDA 2.0.

[tylerjereddy]

SciPy will also be 3.6+ only for the next release (1.5.x). NumPy & SciPy have started to remove large amounts of Python 2 compatibility constructs in their master branches now that their Python 2.7 LTS branches have had what should be their final updates/need for backports.

[orbeckst]

For 2.0 I don't have a problem starting with Python >= 3.6.

[IAlibay]

Going to link #2739 (comment) back to here.

My current approach has been to remove six / future things every time I deal with a file. We probably want a means of tracking progress though.

[IAlibay]

As mentioned in #2750 (comment) it might be that we want to chip away at this in stages. Having discussed this with @lilyminium, it might be an idea to break it down per "section" where we cover what is both in package & testsuite:

• analysis Remove py2 things from analysis #2753
• auxiliary Remove py2 things from aux, vis, transform, and others #2762
• core Remove py2 things from core #2756
• lib Remove py2 things from lib #2752
• selections Remove py2 things from aux, vis, transform, and others #2762
• tests Remove py2 things from aux, vis, transform, and others #2762
• topology Remove py2 things from coordinates & topology #2759
• coordinates Remove py2 things from coordinates & topology #2759
• transformations Remove py2 things from aux, vis, transform, and others #2762
• visualisation Remove py2 things from aux, vis, transform, and others #2762
• misc (benchmarks, maintainer, setup scripts, datafiles, etc...) Remove py2 things from aux, vis, transform, and others #2762
• when everything is finished: remove six from setup.py