[ase-users] Discussion: Next release 3.17, then dropping Python2 support

[Ask Hjorth Larsen]

Dear all,

First of all now is a good time to polish things a bit for the 3.17 release.

Also, importantly, Numpy is going to drop Py2 support soon.  Quoting [1]:

  Until December 31, 2018, all NumPy releases will fully support both
Python2 and Python3.
  Starting on January 1, 2019, any new feature releases will support
only Python3.

Supporting Python2 in ASE also makes life more complicated, so we (JJ
and I anyway) were thinking that after ase-3.17, which would hopefully
be soon (there are a lot of new interesting features), we should drop
Py2 support altogether.

Some arbitrarily chosen annoyances with supporting py2:

 * The need to always run tests on both
 * pip and pip3 both want to install the ase command into the same place
    - This interferes with testing in general, and particularly the
automated testing of installation tarballs
 * We cannot use the actual py3 features that we like (e.g. raise from)
 * Too many subtle differences to list (range/xrange, {}.items(),
...), library differences, etc.

Are there any good reasons to continue to support Python2?  It would
seem reasonable to ask sysadmins to upgrade the Python environment on
any supercomputer which is still stuck with Py2.

Best regards
Ask

[1] https://docs.scipy.org/doc/numpy-1.14.0/neps/dropping-python2.7-proposal.html