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LSP for Python and Scala

Dimarts 21 Maig 2019

If you have have known me for any length of time you'll know I write mostly Python and Scala lately (Rust is getting into the mix slowly). And you should know, I am a heavy emacs user. I have been using emacs for close to 15 years, for the past 3 my emacs of choice has been spacemacs. I used to have a very long, customised and complex .emacs and with spacemacs I get a mostly-batteries-included package. That's nice after a while, and I also have gotten really proficient at using evil.

One problem of using emacs is the integration with some languages. If you write Scala, with IntelliJ you get super-fancy completion, refactoring, code analysis, jump-to-definition... Many goodies. In emacs, the best-in-class system used to be ensime. It worked, but it was not really supported for spacemacs (since I'm an old emacs user I could play around that), but the main issue was that my old MacBook was short on memory for running ensime and a lot more. So, I wrote most of my Scala code in hardcode mode. No completion, documentation or jump to definition.

This is why I learnt how to set up GNU global, jump to definition is just too handy. Luckily, the people at Scala Center not only are smart, but also try to improve developer experience, and had been working in a language server for Scala for a while, called metals. I got it working recently, and it's great. You get documentation on hover, error messages, jump to definition. Oh, I forgot to mention, the language server protocol is an invention from Microsoft to standardise how editors handle language completions and all that. They probably introduced it for Visual Studio Code (I actually use it from time to time, it has some remote pair programming capabilities I'll talk someday), but now it's extending across all editors.

After using LSP in emacs for Scala for a while I decided to set it up for Python as well, in preparation for our next PyBCN podcast, about tools we use. I was pretty happy with the completions I was getting, but semantic completions from a language server are usually better. So far, lsp with python is ok. Oh, you'll see screenshots at the end!

You'll need to install the language server. I usually have a high level Python environment with all my tools, for things I am just starting to work on:

pyenv virtualenv 3.7.1 tools
pyenv activate 3.7.1/envs/tools
pip install "python-language-server[all]" bpython mypy flake8

After this, some configuration is needed in emacs. Here you can find parts of my configuration, commented. These sit in my dotspacemacs/user-config
;; First come the configurations for Scala language server 
;; thingies. sbt is the Scala build system.

(use-package scala-mode
:mode "\\.s\\(cala\\|bt\\)$")

(use-package sbt-mode
:commands sbt-start sbt-command

;; This is the main mode for LSP
(use-package lsp-mode
:init (setq lsp-prefer-flymake nil)
:ensure t)

;; This makes imenu-lsp-minor-mode available. This minor mode
;; will show a table of contents of methods, classes, variables.
;; You can configure it to be on the left by using `configure`

(add-hook 'lsp-after-open-hook 'lsp-enable-imenu)

;; lsp-ui enables the fancy showing of documentation, error
;; messages and type hints
(use-package lsp-ui
:ensure t
(setq lsp-ui-sideline-ignore-duplicate t)
(add-hook 'lsp-mode-hook 'lsp-ui-mode))

;; company is the best autocompletion system for emacs (probably)
;; and this uses the language server to provide semantic completions

(use-package company-lsp
:commands company-lsp
(push 'company-lsp company-backends))

;; I use pyenv to handle my virtual environments, so when I enable
;; pyenv in a Python buffer, it will trigger lsp. Otherwise, it
;; will use the old systems (I think based on jedi)

(add-hook 'pyenv-mode-hook 'lsp)

;; Flycheck checks your code and helps show alerts from the linter
(use-package flycheck
:init (global-flycheck-mode))

;; Show flake8 errors in lsp-ui
(defun lsp-set-cfg ()
(let ((lsp-cfg `(:pyls (:configurationSources ("flake8")))))
(lsp--set-configuration lsp-cfg)))

;; Activate that after lsp has started
(add-hook 'lsp-after-initialize-hook 'lsp-set-cfg)

;; I like proper fonts for documentation, in this case I use the
;; Inter font. High legibility, small size

(add-hook 'lsp-ui-doc-frame-hook
(lambda (frame _w)
(set-face-attribute 'default frame

:font "Inter"

:height 140)))

;; Configure lsp-scala after Scala file has been opened
(use-package lsp-scala
:after scala-mode
:demand t
:hook (scala-mode . lsp))

You can see how it looks now.

Traditional completion (non-LSP)
LSP-powered completion. Way more information!

Fancy inlined documentation
Sadly, inlined documentation doesn't look as good as it should: compare with Scala with metals and lsp-scala:

LSP mode in Scala with metals

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Font: Mostly Maths
Més, sobre...: emacs
Ruben Berenguel
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