[cctbxbb] Python 3 and CCP4

[David Waterman]

Hi folks,

It's great to see the momentum building behind the project to migrate to
Python 3. From the roadmap
<https://github.com/cctbx/cctbx_project/wiki/Python-3-roadmap>, there is
clearly still some way to go, but current activity makes me wonder if the
proposed deadline of complete migration by 2020 might be beaten easily.
What is not clear from the roadmap is whether there are any plans for a
post-transition grace period, in which we have switched from writing
forwards compatible Python 2 code to writing backwards compatible Python 3
code. Has this been thought about? Or will it be the case that the first
commit of Py3-only code seals the deal?

I ask because CCP4 has to support a large number of Python projects and
these have not all considered the Python 3 switch nearly as seriously as
cctbx yet. What I would like to request is a reasonable period of backwards
compatibility is ensured to allow other CCP4 projects to catch up, and that
this protected period is enshrined within the roadmap or developer
guidelines. What do you think?

Best wishes
-- David
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