use compiler to switch to sage (instead of pyton, say, by :comp python)

compiler/sage.vim

@@ -0,0 +1,18 @@
+if exists("current_compiler") | finish | endif
+let current_compiler = "sage"
+
+if exists(":CompilerSet") != 2		" older Vim always used :setlocal
+  command -nargs=* CompilerSet setlocal <args>
+endif
+
+let s:cpo_save = &cpo
+set cpo&vim
+
+setlocal makeprg=sage\ -b\ &&\ sage\ -t\ %:S
+setlocal errorformat<
+
+silent CompilerSet makeprg
+silent CompilerSet errorformat
+
+let &cpo = s:cpo_save
+unlet s:cpo_save

ftplugin/sage.vim

@@ -5,7 +5,9 @@ if version > 600
 endif
 
 " Sage settings (from Franco Saliola)
-autocmd Filetype sage set tabstop=2|set shiftwidth=2|set expandtab
-autocmd FileType sage set makeprg=sage\ -b\ &&\ sage\ -t\ %
+setlocal tabstop=2 shiftwidth=2 expandtab
+compiler sage
 
+let b:undo_ftplugin =  'setlocal tabstop< shiftwidth< expandtab<'
+let b:undo_ftplugin .= '| setlocal makeprg< errorformat<'
 let b:did_ftplugin = 1