Small issue when running Python code straight from VIM

Question

I want to be able to set up the hot key \\ to be able to write and run python script from VIM without each time having to type in

:w
:! python3 file.py

What I have done so far is pasted the following into my vimrc file:

"{{{ The following is for sourcing command.vim whenever exists.
" Function to source only if the file command.vim exists
" https://devel.tech/snippets/n/vIIMz8vZ/load-vim-source-files-only-if-they-exist
"
function! SourceIfExists(file)
  if filereadable(expand(a:file))
    echom a:file . " is about to be sourced."
    exe 'source' a:file
  endif
endfunction
autocmd BufEnter * call SourceIfExists("command.vim")
" }}}

and then in the same directory that my python file is, I create a file named command.vim and paste the following inside that file:

noremap <leader><leader> :w <cr> :!python3 % &<cr>

Now this almost works perfectly except for the following issue. Suppose I want to run the following script, which I'll call file.py:

import numpy as np

x = np.linspace(0, 5, 6)
y = np.linspace(6, 10, 5)

print('{}\n{}'.format(x, y))

If I run this the standard way by using :! python3 file.py then the output is the following:

[0. 1. 2. 3. 4. 5.]
[ 6.  7.  8.  9. 10.]

Press ENTER or type command to continue

But, if I use the \\ method to run the same script, I get the following output:

Press ENTER or type command to continue[0. 1. 2. 3. 4. 5.]
                                                          [ 6.  7.  8.  9. 10.]

and you can see that the output of the script using \\ is not formatted nicely like it is when using the standard command :! python3 file.py.

Anyone know how to fix this?

Answer 1

When you run this:

:! python3 file.py

Vim will run python3 file.py, display the output in the internal pager as the program outputs it, and asks you to press enter to continue when the program exits.

Your mapping is very similar, but not identical as it adds & at the end of the command:

noremap <leader><leader> :w <cr> :!python3 % &<cr>

The & instructs the shell to run the program in the "background", and immediately return control to the shell. With this you get the "Press ENTER or type command to continue" first because the program returns immediately, but it keeps running in the background and still outputs text whenever it's ready, which will interfere with Vim's UI drawing.

This is not a "Vim feature" but rather a "shell feature", for example:

% sleep 2; echo 'hello'
% (sleep 2; echo 'hello') &

In the first command, the shell will wait two seconds and then outputs hello; you can't really do anything in the meanwhile. In the second command it returns control to the shell and you can continue typing new commands, and it will echo hello after two seconds, which may be in the middle of the output of another command (the ( and ) in the second command just group the two commands together.)

You rarely want to use :!... & unless you have a specific reason; simply dropping the & should fix it:

noremap <leader><leader> :w <cr> :!python3 %<cr>