How to set a local variable for a plugin in a BufNewFile autocmd

Question

I currently have

autocmd BufRead *.sage
        \ set filetype=python |
        \ let b:syntastic_skip_checks=1

which works as intended (sage files are treated as Python, but with Syntastic disabled), and I would like to add BufNewFile (i.e. autocmd BufRead,BufNewFile) to it, but when I do, vim complains due to the b:syntastic_skip_checks buffer-local variable not yet existing at the time the BufNewFile event is triggered. Is there a way to either force Syntastic to load in that buffer before the autocmd executes, or to defer setting that variable until after the buffer is fully initialised?
Thanks!

Answer 1

Despite you can use autocmds to set the 'filetype', it is better to create a ~/.vim/filetype.vim, as mentioned in :help 43.2 and explained in Vim FAQ 26.8:

A better alternative to the above approach is to create a filetype.vim file in the ~/.vim directory (or in one of the directories specified in the 'runtimepath' option) and add the following lines:

" my filetype file
if exists("did_load_filetypes")
    finish
endif
augroup filetypedetect
    au! BufRead,BufNewFile *.x       setfiletype c
augroup END

Then you could use the python filetype plugin (check :help filetype-plugin if you aren't using it) to select the correct configuration for the sage files -- it is sourced when the filetype is being set, so the buffer will be fully initialised:

" at ~/.vim/ftplugin/python.vim:
if expand("%:e") == "sage"
   let b:syntastic_skip_checks=1
endif