Should each `ftplugin / name.vim` define` b: undo_ftplugin`?

Question

Some scripts in $VIMRUNTIME/ftplugin/ (e.g. python.vim and ada.vim ) do not define b:undo_ftplugin . The default value of cpo is aABceFs .

When I set ft=python , then set ft=css . $VIMRUNTIME/ftplugin/css.vim finish immediately. And omnifunc=pythoncomplete#Complete all the time.

Should each ftplugin/name.vim define b:undo_ftplugin ?

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This is /usr/share/vim/vim73/ftplugin.vim :

 " Vim support file to switch on loading plugins for file types " " Maintainer: Bram Moolenaar < [e-mail] > " Last change: 2006 Apr 30 if exists("did_load_ftplugin") finish endif let did_load_ftplugin = 1 augroup filetypeplugin au FileType * call s:LoadFTPlugin() func! s:LoadFTPlugin() if exists("b:undo_ftplugin") exe b:undo_ftplugin unlet! b:undo_ftplugin b:did_ftplugin endif let s = expand("<amatch>") if s != "" if &cpo =~# "S" && exists("b:did_ftplugin") " In compatible mode options are reset to the global values, need to " set the local values also when a plugin was already used. unlet b:did_ftplugin endif " When there is a dot it is used to separate filetype names. Thus for " "aaa.bbb" load "aaa" and then "bbb". for name in split(s, '\.') exe 'runtime! ftplugin/' . name . '.vim ftplugin/' . name . '_*.vim ftplugin/' . name . '/*.vim' endfor endif endfunc augroup END 

This is /usr/share/vim/vim73/ftplugin/css.vim :

 " Vim filetype plugin file " Language: CSS " Maintainer: Nikolai Weibull < [e-mail] > " Latest Revision: 2008-07-09 if exists("b:did_ftplugin") finish endif let b:did_ftplugin = 1 let s:cpo_save = &cpo set cpo&vim let b:undo_ftplugin = "setl com< cms< inc< fo< ofu<" setlocal comments=s1:/*,mb:*,ex:*/ commentstring& setlocal formatoptions-=t formatoptions+=croql setlocal omnifunc=csscomplete#CompleteCSS let &l:include = '^\s*@import\s\+\%(url(\)\=' let &cpo = s:cpo_save unlet s:cpo_save 

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If I set ft=python , then set ft=css . Vim cannot pass this test:

 if &cpo =~# "S" && exists("b:did_ftplugin") 

b:did_ftplugin not deleted, so ftplugin/css.vim exits immediately.

Answer 1

:help undo_ftplugin mentioned:

When the user does ": setfiletype xyz", the effect of the previous file type should be undone.

Notice that he says “must,” not “must." But according to the implementation

 func! s:LoadFTPlugin() if exists("b:undo_ftplugin") exe b:undo_ftplugin unlet! b:undo_ftplugin b:did_ftplugin endif 

ftplugin must define b:undo_ftplugin , or its file type parameters can no longer be changed with :setf . I think the documentation should point this out, and all ftplugins really should set b:undo_ftplugin (if only the no-op value is empty).