Visual Studio Code + Official Python Plugin
Here you see an overview of its current Python features:
https://code.visualstudio.com/docs/languages/python
Chocolat
http://chocolatapp.com
It's lightweight and offers Code Completion. Costs money.
EDIT:
Apparently Chocolat was an interesting option in 2013 but since then many others came up and development stalled. Nowadays I recommend Visual Studio Code + Python Plugin.
