Here is a minimal vimrc that I used to test the omni completion issue seen in vim-lsp issue #578

.vimrc

" install: curl https://raw.githubusercontent.com/prabirshrestha/vim-lsp/master/minimal.vimrc -o /tmp/minimal.vimrc
" uninstall: rm /tmp/plug.vim && rm -rf /tmp/plugged
" run vim/neovim with minimal.vimrc
" vim -u minimal.vimrc
" :PlugInstall

set nocompatible hidden laststatus=2

if !filereadable('/tmp/plug.vim')
  silent !curl --insecure -fLo /tmp/plug.vim
    \ https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
endif

source /tmp/plug.vim

call plug#begin('/tmp/plugged')
Plug 'prabirshrestha/async.vim'
Plug 'prabirshrestha/vim-lsp'
call plug#end()

autocmd FileType python setlocal omnifunc=lsp#complete
if executable('pyls')
    " pip install python-language-server
    au User lsp_setup call lsp#register_server({
        \ 'name': 'pyls',
        \ 'cmd': {server_info->['pyls']},
        \ 'whitelist': ['python'],
        \ })
endif