This post is about configuring Elpy
and Emacs to auto-format Python buffers using Black
for a specific project (i.e., using Directory Variables).
Note: for a while, I used firestarter to accomplish this,
but that approach seemed clunky for what I needed.
I mention it here as another useful tool for run-on-save in Emacs
Configuring the Elpy RPC virtualenv
When using Elpy, there are two virtualenvs (or alternatively, conda environments)
involved:
- The virtualenv containing Python packages needed by your project
(we’ll refer to this as the working environment).
- The Elpy RPC virtualenv, where packages used by Elpy (such as
jedi and black) are installed.
Personally, I find it better practice to just use the working environment
as the RPC environment. This allows me to control the versions of formatters
like black on a per-project basis. But, it also means having to install
them in each of my working environments.
;; init.el
;; Use the working environment as the RPC environment
(setq elpy-rpc-virtualenv-path 'current)
If you do this, you must activate the working environment in Emacs
at the start of each session.
You can do this with M-x pyvenv-activate and pointing to the appropriate directory.
At this point, you should run M-x elpy-config to make sure
Elpy has found the right virtualenv,
and also is able to find the black formatter.
Create (or update) .dir-locals.el
The .dir-locals.el file allows you to define variables
local to a directory and its sub-directories.
This makes it possible to specialize your Emacs configuration for a project,
while keeping a more general configuration across projects.
Place the .dir-locals.el file in your project’s top-level directory,
and add the following entry:
((python-mode . ((eval . (add-hook 'before-save-hook #'elpy-black-fix-code nil t)))))
After a restart of Emacs, run M-x pyvenv-activate again,
and navigate to your project directory.
You may be warned by Emacs about the directory local variable
being possibly unsafe.
Type ! to silence this warning.
That’s it! Python buffers in this project should now be formatted
when you save them:
Before save:
import os
a = [1, 2, 3]
def func(x,y = 3):
z= x+ y
return z
After save:
import os
a = [1, 2, 3]
def func(x, y=3):
z = x + y
return z
Note that Elpy will respect the Black configuration provided in
a pyproject.toml placed in the project root directory.