Update 2021-07-15: Note that pyls has been deprecated. There is new fork of it called pylsp, which is maintained by the community.

After using deoplete along with deoplete-jedi for more than two years, I have finally decided to try something new for code completion. Yeah, you guess it, the Language Server Protocol (LSP in short).

LSP is not new. It has been around for several years. Especially with the official LSP support in Neovim core, I think it is time to go for LSP.

To use LSP, there are two parts involved. First, we need to install a language server for the language we use. Second, we need to use a plugin that acts as an LSP client that communicates with this server.

There are quite a few LSP clients out there. Some of the most popular ones are:

I want something simple to set up, powerful, and yet not too monolithic. So I choose vim-lsp for a try. Here is how to configure it for Python auto-completion and more.

First, we need to install a language server for Python. pyls is good choice. First, install pyls:

pip install "python-language-server[all]"

After that, we need to install several plugins and configure it a bit:

Plug 'prabirshrestha/vim-lsp'
Plug 'Shougo/deoplete.nvim'
Plug 'lighttiger2505/deoplete-vim-lsp'

" settings for pyls
if executable('pyls')
    " pip install python-language-server
    au User lsp_setup call lsp#register_server({
        \ 'name': 'pyls',
        \ 'cmd': {server_info->['pyls']},
        \ 'allowlist': ['python'],
        \ })

function! s:on_lsp_buffer_enabled() abort
    " use omnifunc if you are fine with it.
    " setlocal omnifunc=lsp#complete
    if exists('+tagfunc') | setlocal tagfunc=lsp#tagfunc | endif
    " some mappings to use, tweak as you wish.
    nmap <buffer> gd <plug>(lsp-definition)
    nmap <buffer> gr <plug>(lsp-references)
    nmap <buffer> gi <plug>(lsp-implementation)
    nmap <buffer> gt <plug>(lsp-type-definition)
    nmap <buffer> <leader>rn <plug>(lsp-rename)
    nmap <buffer> [g <Plug>(lsp-previous-diagnostic)
    nmap <buffer> ]g <Plug>(lsp-next-diagnostic)
    nmap <buffer> K <plug>(lsp-hover)

augroup lsp_install
    " call s:on_lsp_buffer_enabled only for languages that has the server registered.
    autocmd User lsp_buffer_enabled call s:on_lsp_buffer_enabled()
augroup END

By default, vim-lsp does not provide auto-completion feature, you need to use omnifunc and trigger completion yourself. If you like this, uncomment the following line in above config:

    " setlocal omnifunc=lsp#complete

Then trigger completion via CTRL-X_CTRL-O (Ctrl-X followed by Ctrl-O) in insert mode.

Plugin deoplete.nvim and lighttiger2505/deoplete-vim-lsp are used to provide IDE-like auto-completion feature.

There are other plugins that provide auto-completion support based on vim-lsp, for example, asyncomplete.vim along with asyncomplete-lsp.vim.

Plug 'prabirshrestha/asyncomplete.vim'
Plug 'prabirshrestha/asyncomplete-lsp.vim'

vim-lsp has a lot of options to tweak, some of the configs are:

" whether to enable diagnostics for vim-lsp (we may want to use ALE for other
" plugins for that.
let g:lsp_diagnostics_enabled = 1

" show diagnostic signs
let g:lsp_signs_enabled = 1
let g:lsp_signs_error = {'text': '✗'}
let g:lsp_signs_warning = {'text': '!'}
let g:lsp_highlights_enabled = 0

" Do not use virtual text, they are far too obtrusive.
let g:lsp_virtual_text_enabled = 0
" echo a diagnostic message at cursor position
let g:lsp_diagnostics_echo_cursor = 0
" show diagnostic in floating window
let g:lsp_diagnostics_float_cursor = 1
" whether to enable highlight a symbol and its references
let g:lsp_highlight_references_enabled = 1
let g:lsp_preview_max_width = 80

See :h vim-lsp for more details.

After all these settings, we should get a Python auto-completion, diagnostics etc. using LSP.