Just a few days ago, the built-in Language Server Protocol support has been merged into Neovim master.

This post is a hands-on experience of setting up LSP support for Python with the python-lsp-server.

Note that settings here may be obsolete due to rapid evolving of the built-in lsp.

Install Neovim

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To try the built-in LSP, we have to install Neovim master build release or build Neovim from source. The easiest way is to install Neovim nightly release.

Install nvim-lspconfig

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To ease configuration, the contributor to LSP has created the nvim-lspconfig plugin. We can use vim-plug to install this plugin:

Plug 'neovim/nvim-lspconfig'

Configuration for Python

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First, we need to install python-lsp-server. We can install python-lsp-server and all its dependencies with the following command:

pip install 'python-lsp-server[all]'

Open init.vim and add the following settings:

lua << EOF
require("lspconfig").pylsp.setup{}
EOF

set completeopt-=preview

" use omni completion provided by lsp
autocmd Filetype python setlocal omnifunc=v:lua.vim.lsp.omnifunc

Then open a Python source file. Note that the built-in LSP does not support auto-completion like what is provided by deoplete. To invoke completion, we need to press <C-X><C-O>¹. If all goes well, you should be able to see a completion menu with all the completion items.

The auto-completion experience

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Neovim may hang or lag behind when you start completion since all the compeltion items will be shown in the completion menu and the number of items may be huge. There is also no auto-completion like what is provided by other more mature completion engines. It may take a while before you can really use this for your daily work. Right now, I suggest you stick to your current completion engines.

Useful mappings

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There are other functions provided by LSP which you may find useful. You can add the following mappings for those functions:

nnoremap <silent> gd    <cmd>lua vim.lsp.buf.declaration()<CR>
nnoremap <silent> <c-]> <cmd>lua vim.lsp.buf.definition()<CR>
nnoremap <silent> K     <cmd>lua vim.lsp.buf.hover()<CR>
nnoremap <silent> gD    <cmd>lua vim.lsp.buf.implementation()<CR>
nnoremap <silent> <c-k> <cmd>lua vim.lsp.buf.signature_help()<CR>
nnoremap <silent> 1gD   <cmd>lua vim.lsp.buf.type_definition()<CR>
nnoremap <silent> gr    <cmd>lua vim.lsp.buf.references()<CR>
nnoremap <silent> g0    <cmd>lua vim.lsp.buf.document_symbol()<CR>
nnoremap <silent> gW    <cmd>lua vim.lsp.buf.workspace_symbol()<CR>

Because some of the methods are not supported by a particular lanuage server. So not all these mappings will work as expected. You may encounter errors when you use these mappings if the language server does not implement such features.

helper plugins

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Although we can use the LSP feature with the above minimum configuration. The user experience is not great, at least worse than Deoplete. There are plugins made around LSP to make your life easier and more comfortable.

The plugin completion-nvim and diagnostic-nvim are two such plugins. completion-nvim provides auto-completion for LSP, and diagnostic-nvim provides better disgnostic experience.

The minimal and complete config is list below:

" Note we need to install these plugins using PlugInstall command before using them.
call plug#begin('~/.local/share/nvim/plugged')
Plug 'neovim/nvim-lsp'
Plug 'nvim-lua/completion-nvim'
Plug 'nvim-lua/diagnostic-nvim'
Plug 'lifepillar/vim-gruvbox8'
call plug#end()

lua << EOF
require'nvim_lsp'.pyls.setup{}
EOF

set completeopt-=preview

" use omni completion provided by lsp
autocmd Filetype python setlocal omnifunc=v:lua.vim.lsp.omnifunc

" some shortcuts
nnoremap <silent> gd    <cmd>lua vim.lsp.buf.declaration()<CR>
nnoremap <silent> <c-]> <cmd>lua vim.lsp.buf.definition()<CR>
nnoremap <silent> K     <cmd>lua vim.lsp.buf.hover()<CR>
nnoremap <silent> gD    <cmd>lua vim.lsp.buf.implementation()<CR>
nnoremap <silent> <c-k> <cmd>lua vim.lsp.buf.signature_help()<CR>
nnoremap <silent> 1gD   <cmd>lua vim.lsp.buf.type_definition()<CR>
nnoremap <silent> gr    <cmd>lua vim.lsp.buf.references()<CR>
nnoremap <silent> g0    <cmd>lua vim.lsp.buf.document_symbol()<CR>
nnoremap <silent> gW    <cmd>lua vim.lsp.buf.workspace_symbol()<CR>

set completeopt=menuone,noinsert,noselect

lua << EOF
local on_attach_vim = function(client)
  require'completion'.on_attach(client)
  require'diagnostic'.on_attach(client)
end
require'nvim_lsp'.pyls.setup{on_attach=on_attach_vim}
EOF

let g:diagnostic_enable_virtual_text = 1
let g:diagnostic_enable_underline = 0
let g:diagnostic_auto_popup_while_jump = 1
let g:diagnostic_insert_delay = 1

colorscheme gruvbox8_hard

A good plugin to install is nvim-compe which provides auto-completion for the built-in lsp.

References

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• https://github.com/neovim/nvim-lspconfig#nvim-lspconfig
• https://www.reddit.com/r/neovim/comments/dx098o/github_neovimnvimlsp_common_configurations_for/
• https://www.reddit.com/r/neovim/comments/dvd8sz/builtin_language_server_with_my_preferred_style/