I know you're looking for open source in particular but I'm a PyCharm convert from Atom / Nuclide / Atom IDE and couldn't be happier. Atom didn't come close in

I know you're looking for open source in particular but I'm a PyCharm convert from Atom / Nuclide / Atom IDE and couldn't be happier. Atom didn't come close in terms of functionality, even with a million plugins. There was also a lot more fiddling to get it to work the way I wanted whereas PyCharm generally just does the right thing (though it's very customizable).