This is why PEP8 exists and people are supposed to follow it. Additionally, mixed spaces-and-tabs is a bug that causes poor readability - there's plenty of people who have their editors set to have tabstops as 2 or 3 spaces rather than 4, and your code will look wrong in their editors.
If you think there's any issues with mixed tabs and spaces in your codebase, Python provides a tool to find out - tabnanny. Additionally, any context-aware code editor will display a warning. (If you're using Sublime Text, the plugin to use is SublimeLinter - I assume there's similar tools for vim and emacs.) You can even set spaces and tabs to display differently in many editors!
It puts some icons in the margins, but I can't figure out how to get an explanation of those icons. I see a bunch of !s next to my import statements.
I encountered a non-obvious Python bug that was caused by mixed spaces-and-tabs: to my human eyes, the indentation looked fine - the same as previous line, but the interpreter saw a different level of indentation.