Python accepts both tabs and spaces. Code that mixes tab and spaces can lead to problematic issues. Especially beginners who are new to python can be confused if they copy some code and it doesn't work as they expected because of issues of invisible whitespace.

Beginners are also more likely to use the editor that comes with the IDLE instead of using a more specialised editor. 

If the IDLE would highlite the fact that tabs are used instead of spaces, it would be easier to spot the issue. I therefore suggest that the IDLE highlites tabs both in the shell mode and the editor mode.

Possible ways to highlite is to have a light grey: 
<--> 
» (at the beginning of the tab)
PyCharm style error underlining