How do you set vimrc to run a command on startup, only if one window is being opened?

Question

I want vim to open Tagbar with :TagbarToggle when vim is starting up, but only:

• If the file is Python
• If it's not vimdiff
• If I'm not using vim -O file1.py file2.py to open multiple windows

I would expect this to work:

if ! &diff && winnr('$') == 1    
    autocmd FileType python :TagbarToggle
endif

Especially since :echo winnr('$') in vim does show the number of windows currently open, but adding echo winnr('$') to vimrc always returns 1, even if multiple windows are being opened with vim -O.

Is there a different way to check in vimrc if multiple windows are being opened?

I'm pretty sure I can hack together a solution based on timers (timer_start), where it waits for vim to open a hard-coded amount of time before checking winnr('$'), but that seems dumb. On slower systems, like a raspberry pi (which I use), the hardcoded value would need to be longer to adjust for longer startup time, but a longer timer on a faster system would be noticeable (the files would open, and half a second later, tagbar would open, which is annoying).

Answer 1

You can use a VimEnter auto-command, which only runs at Vim initialization, but after it has loaded all plug-ins and opened all buffers.

I just tested this combination below. It behaves the way you describe your requirements for the specific use cases you mentioned.

autocmd VimEnter * ++once if !&diff && winnr('$') == 1 && &filetype ==# 'python' | execute 'TagbarToggle' | endif

As suggested by @Matt, we can check for bufnr('$') == 1 instead of winnr('$') == 1 to also cover the case you're opening multiple files in separate tabs with vim -p (but maybe you do want to toggle the tagbar in that case?) I also updated my answer to include a ++once modifier on the autocmd, to avoid accumulating multiple copies of the autocmd when you reload your vimrc (source it again.)

As pointed out by @B.Layer, it might be possible to use a match for *.py on the VimEnter event, so you could use autocmd VimEnter *.py and skip the check for &filetype ==# 'python'

The :execute in the call to :TagbarToggle is to prevent that user-defined command to take the | as an argument. By default, user-defined commands take the bar as an argument, unless they've been carefully defined to allow it as a separator. Using :execute to call it is the standard way to work around this issue.