Version 3.8 or newer:
To show line numbers in the current window, go to Options
and click Show Line Numbers
.
To show them automatically, go to Options
> Configure IDLE
> General
and check the Show line numbers in new windows
box.
Version 3.7 or older:
Unfortunately there is not an option to display line numbers in IDLE although there is an enhancement request open for this.
However, there are a couple of ways to work around this:
Under the edit menu there is a go to line option (there is a default shortcut of Alt+G for this).
There is a display at the bottom right which tells you your current line number / position on the line: