how to override system ftplugin settings with user custom ?

I set one option omnifunc in `~/.vim/ftplugin/python/python.vim` like this:
setlocal omnifunc=3Dpython3complete#Complete
But this can not work.
When I open a python file. I checked omnifunc setting with command:
`:verbose set omnifunc?`
I get output:
omnifunc=3Dpythoncomplete
Last set /usr/share/vim/vim73/ftplugin/python.vim

I want to override the system omnifunc setting with my own setting:
setlocal omnifunc=3Dpython3complete#Complete

Does anybody know why my custom option can not work ?

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