How to filter lines only if the filter is successful?

Question

I have the following lines in ~/.vim/ftplugin/python.vim:

" yapf integration
function FormatFile()
  :normal mp
  0,$!yapf
  :normal `p
endfunction
autocmd BufWritePre *.py call FormatFile()

Thus, whenever I save a .py file its contents are filtered through yapf and automatically consistently formatted. However, when there is a syntax error in my file yapf outputs a traceback instead of formatted code and my whole buffer gets replaced by that traceback. This is not desirable as other plugins would inform me about the error less intrusively.

I know that yapf exits with exit code 0 when it successfully outputs formatted code and exits with exit code 1 when it encounters a syntax error. Can I use that fact or any other trick to replace my buffer with the filter output ONLY when the filter is successful?

Answer 1

I pieced together this workaround from lots of stackexchange posts and vim wiki content:

" yapf integration
function FormatFile()
  let buffer_content = join(getline(1, '$'), "\n")
  let filter_output = systemlist("yapf", buffer_content)
  if v:shell_error == 0
    let view = winsaveview()
    call deletebufline('.', 1, '$')
    call setline(1, filter_output)
    call winrestview(view)
  endif
endfunction
autocmd BufWritePre *.py call FormatFile()