> accidentally mix tabs and spaces, which can get... ugly.

This is trivially solved. You can find and replace with sed[0] if you want to be fancy, or just use your favorite text editor/IDE. The teams I've worked on have never had a problem with this. There is always an established convention in a project, and if you're breaking the convention -- e.g. because you prefer tabs -- you're doing it wrong, just by virtue of breaking the convention. If your project is new and you need to set a convention, do that. (also, PEP8 recommends spaces, so there is an authority to reference as a tie-breaker)

[0]: something like: find ./ -type f -exec sed -i -e 's/ / . /g' {} \;