[arp242]

This is because ftplugin/python.vim does:

  if !exists("g:python_recommended_style") || g:python_recommended_style != 0
    " As suggested by PEP8.
    setlocal expandtab tabstop=4 softtabstop=4 shiftwidth=4
  endif

So if you use "set sw=2" then it leaves tabstop and softtabstop at 4.

You can set that g:python_recommended_style to disable it.

Also sw=0 uses the tabstop value, and softtabstop=-1 uses the shiftwidth value.

I agree Vim's behaviour there is a bit annoying and confusing, but it doesn't really have anything to do with tabs vs. spaces. I strongly prefer tabs myself as well by the way.

Even when you DO use tabs Vim will use spaces if sw/ts/sts differ by the way. Try sw=2 and using >>, or sts=2 with noexpandtab.