What would be the difference between these two commands?
autocmd FileType python nnoremap <buffer> <localleader>d ihello!
And:
autocmd FileType python nnoremap <localleader>d ihello!
One references <buffer> and one does not. To me they act the same in my testing of the two in different splits and then different windows, so was wondering why <buffer> needed to be included at all, but perhaps I'm not understanding why it's really needed.
I've already answered this question you've asked two days ago here
Without
<buffer>, if you open a python file after an html one, either you'll see<option-space>from html still working, or you'll override<option-space>with a new one (if you have another mapping dedicated to python), even in the previous html buffer. Without<buffer>, mappings and abbreviations are global: shared between all buffers
To fully appreciate what happens, try the following
autocmd FileType python iab for for :<left>
autocmd FileType vim iab for for<cr>endfor<up>
And then,
" open a python buffer
e foo.py
" type for + space
" -> should be ok
" open a vim buffer
sp foo.vim
" type for + space
" -> should be OK
" go back to the python buffer
sb foo.py " sb != sp
" type for + space
" -> you should see the vimscript snippet instead of the python one
" open a new python buffer
sp bar.py
" type for + space
" -> it works again!
" if you go back to foo.vim,
sb foo.vim " sb != sp
" -> this time, this is the python snippet..
Non-<buffer> definitions are global, and Filetype event is triggered once per buffer.
=> the right way to do it is to use buffer-specific mappings, abbreviations, commands, variables... for things that are specific to a filetype.
And last thing, these things are not defined at a semantic level: while the even is Filetype, the definition is buffer-related, not filetype-related, as seen in this other Q/A of yours https://vi.stackexchange.com/a/25865/626.
<buffer> (left) -- imgur.com/a/VIkPgYI