Wrong Python Binary Used for RPC Virtual Environment

[drgillis]

Summary

Elpy sometimes locates the wrong Python executable when creating the virtual environment. In my case, it finds the python3 binary in /usr/bin/ rather than /usr/local/bin, even though the latter is higher in my path (and in my exec-path variable).

The problem seems to be in the function elpy-rpc-get-virtualenv-path in elpy-rpc.el. The let binds exec-path to its reverse, which causes it to search lower priority directories before higher priority ones when (executable-find elpy-rpc-python-command) is run. I looked at the pull-request from when this code was added to elpy (#1682), and I believe that exec-path is being reversed here to locate a system executable in the event that a virtual environment is active.

~~One possible solution is to create a custom variable where the user can define what directory (or directories) they want to be searched for their python binary. This gives the user a fall-back solution if the wrong one is found after exec-path is reversed.~~ I now see that the variable elpy-rpc-python-command can be used in this manner. However, this fact seems to be non-obvious.

As a short-term workaround, I ran (setq exec-path (append exec-path '("/usr/local/bin"))) before running (elpy-config).

Steps to reproduce

1. Set elpy-rpc-python-command to "python3".
2. Delete (or move) the ~/.emacs.d/elpy/ directory.
3. Create two dummy directories and put a different version of the python3 binary in each of them.
4. Add both of these dummy directories to your exec-path, keeping track of the order they are inserted.
5. Run (elpy-config).
6. Notice that the RPC Python is the version of Python in the latter of the two dummy directories in exec-path.

My configuration

OS

macOS 10.15

Result of (elpy-config)

Emacs.............: 28.0.50
Elpy..............: 1.35.0
Virtualenv........: None
Interactive Python: python3 3.9.4 (/usr/local/bin/python3)
RPC virtualenv....: rpc-venv (/Users/drg/.emacs.d/elpy/rpc-venv)
 Python...........: python3 3.7.3 (/Users/drg/.emacs.d/elpy/rpc-venv/bin/python3)
 Jedi.............: Not found
 Rope.............: Not found
 Autopep8.........: Not found
 Yapf.............: Not found
 Black............: Not found
Syntax checker....: flake8 (/usr/local/bin/flake8)

Elpy configuration in my init.el

None