How to let `g:python3_host_prog` read the python path dynamically via `command -v python3` in Neovim?

Question

I'm using python venv to manage my local pip packages. I'd want to let g:python3_host_prog read the result of shell command line command -v python3 dynamically when venv enabled/disabled in Neovim.

However, when I tried to use :

let g:python3_host_prog = expand( system('command -v python3') )

it shows:

:echo has('python3')
0

BTW, it works when using hardcode python3 path.

Answer 1

Okay, the reason is the extra new line \n returned by system(..), check :help press-enter for more details :

So the solutions is to "suppress" the extra content ( new lines ), here are multiple way can handle:

1. via trim() : trim string

mentioned by @D.BenKnoble :

" .vimrc
let g:python3_host_prog = expand( trim(system('command -v python3')) )

• :help trim()

  trim({text} [, {mask} [, {dir}]])

  If {mask} is not given, or is an empty string, {mask} is all characters up to 0x20, which includes Tab, space, NL and CR, plus the non-breaking space character 0xa0

• debug:

  :echo "aabbcc\n"
  
  " trim
  :echo trim( "aabbcc\n" )
  :echo trim( "aabbcc\n", "\n" )
  
  " or even more new lines
  :echo trim( "aabbcc\n\n\n" )
  

2. via systemlist(): return list of system()

thanks @D.BenKnoble

let g:python3_host_prog = expand( systemlist('command -v python3')[0] )

" or
let g:python3_host_prog = expand( get(systemlist('command -v python3'), 0) )

" or with default value if not found from cmd
let g:python3_host_prog = expand( get(systemlist('command -v python3'), 0, '/usr/local/bin/python3') )

• more info

  systemlist('...') == split(system('...'), '\n')
  

• :help systemlist()

  systemlist({cmd} [, {input} [, {keepempty}]])

  Same as |system()|, but returns a |List| with lines (parts of output separated by NL) with NULs transformed into NLs.

• :help get()

  get({list}, {idx} [, {default}])

  Get item {idx} from |List| {list}. When this item is not available return {default}.

3. via substitute(): find and replace

Got clue from https://stackoverflow.com/a/52983182/2940319 and tomtom/tlib_vim :

let g:python3_host_prog = expand( substitute(system('command -v python3'), '\n\+$', '', '') )
"                                            ----------------------------   ------  --
"                                                          |                   |    + replace to ''
"                                                          |                   + search patten: find one or more new line(s)
"                                                          + string

" or you can use two lines:
let python3_dynamic_path = substitute(system('command -v python3'), '\n\+$', '', '')
let g:python3_host_prog = expand(python3_dynamic_path)

• Search and replace

  The :substitute command searches for a text pattern, and replaces it with a text string.

• :help substitute()

  substitute({string}, {pat}, {sub}, {flags})

---

the :checkhealth provider.python result:

==============================================================================
provider.python: require("provider.python.health").check()

Python 3 provider (optional) ~
- Using: g:python3_host_prog = "/Users/marslo/.venv/tools/bin/python3"
- Executable: /Users/marslo/.venv/tools/bin/python3
- Python version: 3.12.3
- pynvim version: 0.5.0
- OK Latest pynvim is installed.

Python virtualenv ~
- $VIRTUAL_ENV is set to: /Users/marslo/.venv/tools
- Python version: 3.12.3
- OK $VIRTUAL_ENV provides :!python.