Debian's Elpy package is configured to use a system-wide Python interpreter
that will be supported after 1 January 2020, when Python 2 will be EOL.
The specific customisations are:

(setq elpy-rpc-pythonpath nil
      python-shell-interpreter "/usr/bin/python3"
      python-shell-interpreter-args "-i"
      elpy-rpc-python-command "/usr/bin/python3")

Customise these variables to enable specific Python versions, or
reenable the normal rules of $PATH by dropping the absolute path. eg:
Specify python3 and let $PATH choose python3 from something like
~/.my_virtualenv2/bin/python3.  The system-wide Python interpreter was
chosen--using an absolute path--to guard against the situation where a
user has installed an additional, possibly experimental Python version
somewhere in their $PATH.  Such a state would have been confusing for
a developer who was new to UNIX-like systems!

Elpy's custom PYTHONPATH is not needed in packages that use dh-python,
and has been disabled.  Previously Elpy unnecessarily searched
site-lisp/elpa/elpy-1.23.0 for its associated Python modules.  This
should not be changed.

An alternative interactive interpreter can be enabled by setting
'python-shell-interpreter' and 'python-shell-interpreter-args' to (eg:
Jupyter console or IPython).  See "Interpreter Setup" section of
Elpy's Documentation for more information.

To restore upstream defaults, add the following to a user's Emacs config:

    (setq python-shell-interpreter "python"
          python-shell-interpreter-args "-i")
          elpy-rpc-python-command "python")

These defaults are useful when moving between virtualenv projects
where some projects use Python 3 and some projects use Python 2.  That
said, at present the Debian package is not built '--with python2' and
so it is likely that Python 2 virtualenvs do not yet work correctly,
because Python 2 modules are not installed to a system-wide PYTHONPATH.
These modules are needed for the RPC Process.  See Concepts->The RPC Process
in 'info elpy' for further information.

 -- Nicholas D Steeves <nsteeves@gmail.com>, Mon,  3 Sep 2018 19:09:35 -0400
